Well, there is no perfect magic wand in these scenarios, but if it is the same service and that service is setup to use design line, then you might be okay with a REVDESIGN. The quality of the model that was produced will determine how much tweaking the resulting design line needs refill properly. If this isn't a system requiring directional fittings (IE drainage), then make sure the service flow type has a value of "Not Set"; it seems to work best.

I am fuzzy on whether drawn parts hold design line button codes from the moment they are initially drawn or constantly refer back to the service template.

If they do retain their instance properties and your design line settings were not setup properly when they were piped piece by piece, then the REVDESIGN may not work very well, but still worth a shot. I would recommend using getting it isolated down to ONLY pipe and fittings and create a design line from that using the REVDESIGN command. From that point you can do an Erase3D and go to Line->Edit dialog to change everything to the desired size. Finally try the Fill3D and hope it works.

If you were not properly configured at the time or even now (not much of anything is filling back in from the above step), then undo back to before that, take the time to configure that service to use design line and try again. If that still doesn't do it, you could try hitting everything with an UPDATEFROMSOURCE, but I haven't a clue if that will actually acquire new button code information from the template on existing items.