@Arrix85
I have been doing some digging into the matter of the portions of the old Principality of Pavia, which were mostly lost during the first half of 18th century.
Without going into minor details, the Lomellina was annexed by the Savoy in 1707, and the OltrePo Pavese went the same way in 1744, together with the county of Vigevano. (see the attached map, from Wikipedia)
During the Napoleonic period, there was a referendum in the OltrePo, and the population voted in favor of returning with Pavia, but the result of the referendum was never implemented. No modification were enacted by the Congress of Vienna.
Following the reorganization of the administrative subdivisions in the kingdom of Sardinia, the Lomellina and the county of Vigevano became part of the Divisione (Department) of Novara, while the OltrePo was split into two Provincie, Voghera and Bobbio.
At the same time, the district of Pavia (in Lombardy-Venetia) was slightly enlarged to the north.
IOTL the big changes came in 1859, following the annexation of Lombardy: Lomellina, Voghera and Bobbio were reunited to Pavia.
ITTL, Piedmont and Lombardy are formally separated, even if both crowns are held by the king of Sardinia in personal union.
As I see the matter, Lomellina and OltrePo make more economical sense under Pavia, and the people who live there appear to share this opinion.
Since Milan is fated to be the big economical winner in Northern Italy, the pressure to rejoin Pavia will increase.
The same is true for Piacenza, which makes more sense under the crown of Lombardy, since the bridge over the Po (at this stage still a barge bridge) is the necessary connection on the road from Milano to Bologna, and will become even more important once the railways from Milano to Bologna and from Bologna to Ancona will be built (without forgetting the Livorno-Parma which is a priority for Tuscany and Lombardy).
IOTL, the 1850s saw the start of a number of railway lines, but always with the heavy interference of Austria, which always subordinated the economical viability of the railways to political constraints. A couple of examples can better clarify the situation: Austria vetoed the construction of a railway line from Civitavecchia to Ancona (to protect the revenues of the Trieste port), and insisted for a connection to Siena; the railway from Milano to Bologna was never considered, since Austria wanted a railway from Mantova to Reggio, then Modena and finally Lucca. To increase the chaos, neither Austria nor the Papal States were financially sound, and the railways never truly started before the Rothschild entered the game.
The priorities changed after 1859: the Milano-Bologna and the Bologna-Ancona became a true priority.
A trestle bridge over the Po was operative by the end of 1861, and at the same time two additional bridges were built near the trestle bridge: a steel bridge and a trestle service bridge parallel to it to bring in materials for the construction of the steel bridge. The first train crossed the Po in 1862 (on the trestle bridge), and the steel bridge was inaugurated in 1865. The Bologna-Ancona was completed in the last quarter of 1861 (after solving some problems for the crossing of the Reno river in Bologna).
ITTL, the priorities will be very similar to those of 1859, but the railway construction will come probably 8 to 10 years earlier (and it is likely there will be more British participation in the design and construction; IOTL, most of the engineering came from France. If Cavour can manage it, the Rothschild will still be involved in the financing, but will not enjoy the dominant position they had IOTL (British and Swiss financing will ensure better competitive terms).
This longish detour into railways (which are a bit of a fixation of mine
) has been necessary to point out that also the interests of Piacenza would more and more linked to Lombardy. I would predict that sometime in the 1850s there will be a new referendum in Lomellina, OltrePo and Piacentino to join Lombardy.