Casimir IV was most successfull (or most lucky) ruler of House of Jagiellon. After 150 years he regained Pomerelia for Poland and thanks to his marriage to Elizabeth of Austria he get additional two crowns for his dynasty-Czech and Hungarian ones. Although in the long run his dynastic policy was failure. Southern adventures of Jagiellons ended soon with disastrous Battle of Mohacs.
ITTL course of history is altered with premature death of Elizabeth of Austria in 1452. As result Casimir needs to look for another wife. IOTL Casimir married her to secure his southern border before war against Teutonic Order ,(although his brother-in-law Ladislas the Posthumous still picked TO side), her claims to Czech and Hungarian thrones were bonus. Although marriage of Casimir was happy one, Casimir initially regretted that he didn't married beautiful Sophia of Pomerania instead-Elizabeth had very deformed spine and face.
Sophia of Pomerania was married before POD, so she is not option ITTL. Casimir could instead marry Anna of Saxony (born 1437), OTL Electress of Brandenburg and niece of Emperor Frederick III. That would improve a bit Casimir's relations with southern neighbours-IOTL Czech King George of Podebrady feared Elizabeth's claims to Czech throne and initially was friendly to Teutonic Order during 13 Years War. He allowed TO to hire Czech mercenaries. Perhaps 13YW would be a bit shorter as result. And then we have Matthias Corvinus. IOTL Matthias attemted to make alliance with Poland and asked for hand of Casimir's oldest daughter. He was refused. Casimir and Elizabeth wanted Hungarian throne for their son, and Elizabeth despised Matthias. She regarded him as usurper and man of low birth, unworthy of her daughter's hand (she called him Vlach, peasan, t and dog). As result Matthias turned against Jagiellons and looked for alliance with Teutonic Knights and Moscow against Casimir.
ITTL situation is different. Not only Casimir's wife has no claims to Czech and Hungarian thrones. Say, that unlike Elizabeth she had just one living son by late 1460s (although next ones could be born later), so there is also no need to secure future of numerous sons. Casimir and Anna have also several daughters and they have no such strong reasons to refuse Matthias one of them. So ITTL Casimir IV and Matthias Corvinus may be good allies (poor Habsburgs
) Jagiellons would not waste resources in the south and Corvinus would not be surronded by enemies from every side, and he could succeede in securing the throne for his descendants from his Jagiellon wife (@Fehérvári should be happy about this). And there is Czech question: IOTL after messing with Pope George of Pdebrady eventually allied with Casimir, promising him Czech throne for his son Vladislaus. ITTL having no claims to Czech throne through his wife and no large bunch of sons, Casimir could instead accept offer of Pope and Corvinus and would join anti-Hussite crusade. IOTL Pope promised him to recognize terms of Peace of Thorn if he joins Crusade. For average 15th century monarch Dynasty was more important thing than nation, so Casimir choosen George's proposition of throne for Vladislaus. ITTL with just one son, who is needed as heir in Cracow and Vilnius, Casimir's dynastic policy would be different. That means Poland could use propably the best opportunity to regain Silesia after it was lost to Czechs: Czech King is heretic excommunicated by Pope and crusade is called against him.
ITTL course of history is altered with premature death of Elizabeth of Austria in 1452. As result Casimir needs to look for another wife. IOTL Casimir married her to secure his southern border before war against Teutonic Order ,(although his brother-in-law Ladislas the Posthumous still picked TO side), her claims to Czech and Hungarian thrones were bonus. Although marriage of Casimir was happy one, Casimir initially regretted that he didn't married beautiful Sophia of Pomerania instead-Elizabeth had very deformed spine and face.
Sophia of Pomerania was married before POD, so she is not option ITTL. Casimir could instead marry Anna of Saxony (born 1437), OTL Electress of Brandenburg and niece of Emperor Frederick III. That would improve a bit Casimir's relations with southern neighbours-IOTL Czech King George of Podebrady feared Elizabeth's claims to Czech throne and initially was friendly to Teutonic Order during 13 Years War. He allowed TO to hire Czech mercenaries. Perhaps 13YW would be a bit shorter as result. And then we have Matthias Corvinus. IOTL Matthias attemted to make alliance with Poland and asked for hand of Casimir's oldest daughter. He was refused. Casimir and Elizabeth wanted Hungarian throne for their son, and Elizabeth despised Matthias. She regarded him as usurper and man of low birth, unworthy of her daughter's hand (she called him Vlach, peasan, t and dog). As result Matthias turned against Jagiellons and looked for alliance with Teutonic Knights and Moscow against Casimir.
ITTL situation is different. Not only Casimir's wife has no claims to Czech and Hungarian thrones. Say, that unlike Elizabeth she had just one living son by late 1460s (although next ones could be born later), so there is also no need to secure future of numerous sons. Casimir and Anna have also several daughters and they have no such strong reasons to refuse Matthias one of them. So ITTL Casimir IV and Matthias Corvinus may be good allies (poor Habsburgs
) Jagiellons would not waste resources in the south and Corvinus would not be surronded by enemies from every side, and he could succeede in securing the throne for his descendants from his Jagiellon wife (@Fehérvári should be happy about this). And there is Czech question: IOTL after messing with Pope George of Pdebrady eventually allied with Casimir, promising him Czech throne for his son Vladislaus. ITTL having no claims to Czech throne through his wife and no large bunch of sons, Casimir could instead accept offer of Pope and Corvinus and would join anti-Hussite crusade. IOTL Pope promised him to recognize terms of Peace of Thorn if he joins Crusade. For average 15th century monarch Dynasty was more important thing than nation, so Casimir choosen George's proposition of throne for Vladislaus. ITTL with just one son, who is needed as heir in Cracow and Vilnius, Casimir's dynastic policy would be different. That means Poland could use propably the best opportunity to regain Silesia after it was lost to Czechs: Czech King is heretic excommunicated by Pope and crusade is called against him.