Hello @BigSASFan ,
yours is a very good and interesting question. Of course, this question would need to be answered, ultimately, by a SAS employee, but I can share my thoughts until then.
SPRE, while similar in many ways to SAS 9.4 to quite a degree, it has also several differences and it cannot be used always as a substitute. SPRE is not a full Base SAS (and SAS modules) product, it misses several modules and many procedures simply won't work. And if your code works using the GRID functionalities (such as SASGSUB, connect, etc), this certainly won't work.
This being said, although it wont help you right now, and only SAS employees can confirm or reject this idea, this perspective might help you in the future to you or to others:
As far as I am aware, SAS Viya 4 comes with an evolved SPRE with more SAS 9 functionality/support, and ideally speaking, in one of the future releases of Viya, SPRE should be a 1:1 supported model of SAS 9, shaping what SAS keeps telling us during the Global Forums about the ideal SAS Platform (no SAS 9, no SAS Viya, but a combination of both together).
My suggestion to you, at this moment, is to either have a separate full DEV/TEST/DR environment (depending on how much you want to pay for licencing), also GRID, but with less Grid nodes and resources (the minimum you need for your DR to work at the minimum required by your business processes), or, if the code does not use GRID functionality, a simple 1x SAS 9.4 server with all licensed products except for Grid itself.
I hope this helps!
Best regards,
Juan