I am going from Vista (I know - hence the reason for doing it) to Win 7, with a custom install to go from 32-bit to 64-bit (so I can put more RAM in).
I think I lose my programmes in doing that.

I will save the Bitcoin qt data (I know where that is stored) as well as backing up the wallet in several places. But again question comes down to when I reinstall the Core client will it a) recosgnise the wallet and be able to access it and b) despite the fact that I have pruned the blockhain down to 2Gb it will still be in synch without having to redownload the whole thing?

If answer to both is yes, I guess I am good to go.
If either is no then I dont think I will run a full node and instead go for something like Electrum, but then the question is will that software recognise and be able to access the Core wallet.dat, or do I have to open an Electrum wallet and send my BTC to it via a transaction (with fees etc)?