Exactly. I expect a lot of people will decide to move to a cheaper part of the EU where their meagre pension will go further. Hey wait a minute...

This is happening right now. I don't have any statistics, but I've seen TV reports (i.e., anecdata) about German retirees moving to Romania or Bulgaria. The biggest issue for them is that they only know German, and the locals in their age class only know their own native tongue. Maybe when people from my generation need to do the same in the future, it will be easier for them because English fluency is/will be more widespread by then.

EDIT: Would be interesting to see how this might affect the economy at large when this gets widespread. There already are quite significant transfers of purchase power across borders within the EU, e.g. (IIRC) the main income source in the Republic of Kosovo is family members working in richer parts of the EU and sending money home. Retirees who receive their pensions from the rich countries, but spend it in the poor countries where they relocate to, would serve as another force in the same direction.