I wish to convert an MDE file to an ACCDE file.
Assume I don't have access to the original MDB file.
The internal structure of MDB/MDE and of ACCDB/ACCDE is completely different.
When you change the file extension from MDE to ACCDE, the internal structure remains MDE as before. It does not matter Access because Access knows how to open the file. However, future versions of Access may fail opening it.
You have not explained Why do you want to change the MDE to ACCDE. If the name change itself meets your needs, it's all fine. But if you want the structure of the file to be updated, this is not the solution.
It seems to be the case that I can just rename the file extension from MDE to ACCDE and everything works properly.
EDIT: naftalip has provided the answer that changing the file extension does nothing to convert the file format and Access simply reads the format as MDE and does nothing to upgrade it.