xbee pro 900 compatibility

Are the XBee-PRO 900 compatible with the XBee-PRO 900HP? I have some of each and don’t seem to be able to get one type to talk to the other, am I missing something or are they just not compatible?

I thought it was a pritty simple question, so nobody knows if the XBee-PRO 900HP is a drop in replacement into a XBee-PRO 900 network?

I think they are compatible. But digi provides a guide to migrate from S3 to S3B modules, http://ftp1.digi.com/support/documentation/S3-to-S3B_Migration.pdf
I think there will be some hardware and software differences between S3 and S3B modules.
XBee 900 - S3
XBee 900 HP - S3B