When I did my TZ playfield swap, I used the "old" glass because I wanted to replace it.
After everything have been removed from the top, I took it out and flipped it on a table, unscrew everything and I slided between the playfield and the under-playfield parts the glass.
Then, I could take out the glass with everyting in place, put aside the playfield and put the new one on the table. Then I put back the glass, and slided it out. All I had to do is to re-assemble the under-playfield, and going forward.