RE:
To clarify, are you saying that the new samples and old samples - playing together on the same pitch - don't blend well? If that's the case, then you'll have to rerecord those samples and I recommend using a tuner such as the vst here: http://www.gvst.co.uk/gtune.htm
The reason they're not blending well is because you either a) didn't record the new samples at the same original pitch as the old samples (either a bit too high or too low), or b) you've changed your mic/environment/maybe different noise removal strategy.
If ever possible, I find it best to try to record a bank all in the same day rather than at different times.