As I read it, the original flights were delayed due to weather & OP would have been stranded in FRA otherwise.
If OP had been rebooked in PE (s)he would have arrived a lot later & with no compensation/only duty of care under EU261 as the reason was weather related.
In this case, in my opinion, OP accepted a voluntary downgrade from PE to Y which allowed him/her to arrive same day, albeit later than planned.
While not communicated as such, it was weither downgrade and fly, or get stranded with a meal voucher & hotel bed, or worst case a stretcher in a corridor at FRA.
Sounds harsh but lesson learned I hope, especially lesson 3 added by Oliver2002.