I had gloomy thoughts about this, but wait, it may not be so bad...it's important to remember that portamento and poly/mono settings are global, used for all programs at once (at least on the non-E! DX7...)So try this : go to the function menu and make sure that portamento is zero and mono/poly is poly; appropriate of course for piano.
This is a situation unique to the DX7.

If your sounds are still not right it's possible that a card edge connector is not fully re-seated. Check everything you did in the process of changing the battery. The thing I don't want to think about is, did the old battery leak? I sure hope not...good luck and let us know how it went.mn