I don't think anyone can predict the impact. However, from Pega's perspective, SQL generation should work on either DB versions. I would expect your Pre-Prod should be in line with Prod env, otherwise, there is no way of testing before moving the code to Prod - any reason what that is case? The biggest impact I can possibly think of is the performance. Oracle 12c is a newer version and supposed to be better - so you could have the situation where everything works fine in lower env and not so well in Prod env. I am also aware of special cases Oracle 12c optimizer performs worse.