I recently bought replacement decals for over mansion but seller didn't include instructions.  Just wondering if old decal has to be removed with heat or put new one on directly over.  

Playfields were never decaled..  What is on the inserts is ink from the silkscreening process.  The inserts were installed PRIOR to silkscreening the playfield and were silkscreened and clearcoated at the same time as the rest of the playfield.

It's better to remove the artwork prior to installing decals.  It's a difficult and time consuming job.  You have to sand off the clearcoat and artwork.  Probably easier if you remove the inserts but that's a big job too.

Once you install the decals, you have to clearcoat the inserts or else the decals will wear off/lift FAST...

Mike