Very, Very, Very hard to do.... and if you want a war changing victory it's nearly impossible.
By the summer of 1863 the AOP was a very good army man for man, and Regiment for Regiment. The leadership Maj to COL, and Non_Comm was solid. And they understood, possiblity better than any Confederate other than Longstreet how to fight on a modern " rifled" battlefield.
Even the Cavalry, look at what Buford did to Heath on day one and Custer to Stuart on Day three.
Tthe majority of the division and Corp commanders were equal to their Confederate peers.
They were just waiting for a commanding general who understood how to use them, and not afraid to stay in the fight the moment Lee pulled a flanking moment.
So to give the Confederate at least a minor tactical victory, without Meade doing something stupid (ie Burnside or Pope), which Meade was to solid to do as long as Washington let him fight his battle or Lee listening to Longstreet and falling back to the ridge line west of town (Seminary Ridge), which wasn't going to happen with R.E. Lee in July 1863.
1. Day one, Have Heth push thru Buford and Reynolds (I Corp), quicker before all of I Corps arrives and deny the AOP Cementary Hill, Ridge and Culp Hill. The next defendable line is Rock Creek and it isn't great, and Hancock (senior after Reynolds is killed, till late Day One) isn't stupid. You get a series of AOP fall backs looking for "good" ground and staying between Lee and Virginia (as well as Washington*), small fights, AONV wins was loses men, eventually I suspect IVO Tanneytown, MD (which if I remember has good ridge lines) the AOP digs in, and we fights, bloodies the AONV to the point it has to advance to the rear (Virginia).
2. Day Two, The rounds tops fall, once again Meade and AOP fall back to Rock Creek hurt but in reasonable order (at this point no manner what Lee thought the majority of the Union Army wasn't going break and run. Longstreets Corp is too beat up taking the Round Tops to close the trap anyway. Once again minor victory for Lee, "He has the field," but so what the AOP beaten, one, maybe two Corps are out of the fight, there are still 5 others... Lee has to advance back to Virginia.
3. Day Three, it's over Lee has to win by Day Two.
So in the end, at most Lee get a tactical victory, I don't see the going any longer.