that's a wise decision! By migration to Informix, you have no more limits. You can start with the Innovator-C Edition which is a real GPL license, zero buck cost, for production.
If you need to upgrade the product, just purchase the right license, upgrade the binaries and that's it. A 10 mn thing. No modification except eventually changing some config parameters to take benefit of upper level functionality.
Regarding the migration, there is an excellent Migration Guide written by Sanjit Chakraborty, from the IBM Informix team. It is composed of 2 parts, which do consider exactly what you are looking for, and more.