Just to comfort people:

I upgraded my wallet to 0.15.1.0 and I did keep my wallet.dat
(I did not need to restore, upgrade was done using my existing wallet.dat)

As my exchange (Bit-Z) does not yet support the new public key format, I transferred some BTX from my exchange account to the old format 1xxx public key.

The expected amount (minus withdrawal fee) arrived in my QT BitCore BTX wallet Version 0.15.1.0 under the new public key format 2xxx. As verified in recent transaction list and blockchain explorer.

So for true hodlers, I see no reason why one should not upgrade to version 0.15.1.0, even if your exchange does not yet support the new address format.