Help me decide. To Retrofit or not to Retrofit.......

I do it only as hobby. (not too much experience)

My cheap "m..ch3" hobby mill had reliability issues on the hardware side.
Went for a 2nd hand alternative, ready to run.

Having time and no pressure made a big difference to a point, where instead of getting frustrated it became fun.

For a business. Considering work time, the "expensive spare" can become the cheapest option quickly.
If the spare is "plug and play" I would probably not rush into a retrofit.

I would follow the approach "spumco" described. Safer and less risk of putting myself into a corner.