If possible, it is far better to keep the same structure and same filetree.
And, if possible…when it’s article.html, keep it html…..
Changing all the files to php can be done, but even with the best techniques you will loose some pr at the end.
I wouldn’t take the risk of loosing the good serps
and save me a lot of work and trouble
and simply change the pages slowly as how they are now.
just my 2 cents