Redistricting and reapportionment are about as political as these things get. What does losing a seat mean? From a sheer mapping point of view, losing one seat and not two means there will have to be some serious redrawing of districts all the way across the state. Every district is going to look pretty different once the map drawing is completed. Even if the main population loss is in Western PA, the redrawn districts here will impact boundaries all the way to NE Pennsylvania.
Mentioned before, but when Pennsylvania was last redistricted following the 2000 census, there was little secret made in the desire to create a district that would swap one traditionally Democratic congressional district for a Republican one in Southwestern Pennsylvania. With just a little help from those darn computers a congressional district was tailor made for then State Senator Tim Murphy who would win the seat. Jeff Toobin discussed the whole history of Pennsylvania redistricting in this article in the New Yorker. In the end the effort may have backfired. According to no less of a conservative source than the Wall Street Journal the process didn't work out too well for Republicans in the long run. What ought to have created were maps favorable toward a Pennsylvania delegation that was overwhelmingly R, the current congress about to be seated has 12 D and 7 R.