Welcome to the forum.
At one time, one COULD Open a PREL (the PrE Project file) in PrPro, with some considerations:
The PrE Project did NOT contain any Themes/Instant Movies, as PrPro did not have those.
The PrE Project did NOT contain any 3rd party Transitions, or Effects, that are not available in PrPro.
The versions of PrE and PrPro are somewhat contemporary, such as PrE 4 and PrPro 2.0.
Now, if the PrPro version is PrPro CC (Creative Cloud), it appears that it cannot Open, or Import PrE PREL files, but no Adobe engineer has confirmed that - the info comes from users, who all say, "No go."
Also, a Mac PREL cannot be Opened by PrE on a PC, so there could well be issues there. Your chances of success might be better, if both programs are Mac versions, and the versions are contemporary, say PrE 10 and PrPro CS 6, both on a Mac.
Good luck, and please let us know how well your migration works.
Thanks for that. I'm considering purchasing Elements (as I can't afford Pro), but would just like to know I could migrate if and when necessary. I've been trying to find an answer via googling before posting this question, but not many answers around. Seems odd to me that the company hasn't addressed this. But apparently not.
If you have PrE on a Mac, or on a PC, note that the PREL files (the Project files) are NOT X-platform. One would only be able to migrate a Mac PrE PREL to another Mac with PrE (best if the versions are the same), and the same for PrE on a PC.
PrPro, as of CS 3, has far fewer issues going X-platform. Premiere (precursor of PrPro, and not PrE) was also X-platform, up through Pr 6.5. In that program, major changes came at Pr 7.0, which was most often seen as PrPro 1.0.
Up through PrPro CS 6, there was a capability to either Open, or Import a PREL (see above for some limitations on that), but as of PrPro CC (Creative Cloud), that capability seems to have been omitted. As this is not a common workflow, reports are limited, but to date, it seems that it cannot be done.
Good luck, and hope that helps with your decision.