SRPs and CALs both don't matter when it comes to mail platform. The only thing that matters is the BES software, you will need to get the BESX software from RIM. If you have a TSupport agreement then it's free, if not, sign up for BES Express but don't use the SRP or CAL that you get. You will get the BES for Exchange, latest version, for free.
Free is always good