After further study of the matter, I have decided to update PHP from 5.16 to 5.2.13 using the Atomicorp Repository. Procedure follows:
wget -q -O - http://www.atomicorp.com/installers/atomic.sh | sh
yum update php php-mcrypt php-mhash php-mssql php-tidy
service httpd restart
As a result I was able to successfully switch to RoundCube without any more trouble, via web install.