Combing PDO with mysql_pconnect connections in one application -- will this degrade performance?

Hi,

We are about to convert all of our queries using mysql_pconnect to 
prepared statements using PDO database connections.  It will take some 
time to convert the hundreds of SQL statements so the plan is to do it 
in phases.  Is is bad idea to have both a mysql_pconnect and a PDO 
connection open for the same application?  Will this doubling of our 
database connections for every page hit degrade performance?   Any tips 
on the best way to transition to PDO/prepared statements with minimal 
disruption?

-Sara