Hello,
First, I suggest reading the following document that discusses a migration scenario from PVST+ to RPVST+:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b0670.shtml
Migrating to RPVST+ will be most probably disruptive to the network operation, therefore it is recommended to plan this operation to a maintenance window. The outages will be caused by all ports on the reconfigured switch to going into Discarding state and negotiating their RSTP roles and states anew.
Please feel free to ask further!
Best regards,
Peter