I've appreciated the DCC hints and cautionary notes that I've read in this forum over the past couple of years.
Converting non-DCC ready locomotives in N Scale is challenging for me. But, I feel a sense of accomplishment now that I have 3 upgrades under my belt using Digitrax DZ125 decoders. (Con-Cor PA1, Lifelike GP-18, and Lifelike FA1)
Some of these observations are repetitive of earlier posters & other DCC how-to sources. They may still offer some help to others considering the jump over to DCC in N scale, specifically thinking about upgrading older locos.
Checking your work along the way saves time. I now temporarily wire the decoder to the engine before I shorten the wires just to check the validity of my scheme. And I check it on a conventional DC test track first. I determined that the Red & Black track power wires on the FA1 conversion needed to be soldered directly to the pick-up wires at the bottom of the loco for a reliable connection for example. There is always a discovery along the way. I didn't do this with my 1st conversion, the PA1 and after hours of trouble-shooting I wished that I had.
I usually underestimate (not on purpose) the space that will be needed by the decoder and its wires. It's easy enough to go back to the Dremel. I suppose it's better to take out too little of the frame rather than carve out too much and loose some performance assisting weight. But, I've yet to get the frame modification correct on the first try.
Only use videos you see on YouTube as a guideline. That's not a criticism of the people who take the time to share their DCC How-To's with the rest of us. It's probable or even likely that you may find an additional point on the frame that needs to be insulated electrically or an easier way of hooking things up. Also, when you're working with an older loco it's possible yours is built differently than the one in the video.
Have patience. I'm not an electronics wiz or a skillful model builder. I've spent anywhere from 4 to 6 hours each converting non-DCC ready locos. But, when everything is buttoned-up and I've had a chance to program in the performance and lighting characteristics it's exciting to see how terrific a reborn engine operates.
Again, thanks to all of the trail blazers and those that have taken the time to share their thoughts on the DCC subject.