Some of this will come down to IMHO, what are the upgrades you want to do? ie change the floorplan......buy a new boat! If it is replace the cushions because the color, foam etc is faded, worn out etc. replace the cushions, who or what is to say that the next boat will not need the same thing.
If the varnish or equal is worn, not shiny, much easier to do this yourself. Next boat, could have the same issue. I could probably say the same thing about other boat parts, somewhere along the line, standing rigging needs replacing, as does running rigging etc.
Some things like running rigging is nice to replace/redesign if you will to a degree, as you can then make changes to what and how you want to the lines to run and where. Color coding also is nice to do.
I've frankly probably redone most of y 25 yr olds boat over the last 5 yrs of ownership, from revarnishing the interior, new cushions, redoing the foam backed vinyl hull/cabin top liner etc. It has cost a few $$$, but I am still into it less than a new boat. Some things came with the boat, that would have cost me any how......
In the end, the boat will go another 5-10 yrs with out too much redoing. I do need to budget in the next few years new standing rigging and another mast step fix that a couple of other local boats have had to do. Whether or not you want to call what I have done retrofitting, or what.....that is another story. Some of the stuff, I probably would have done even if a new one, ie color coding the running rigging, new laminate sails vs dacrons that come with the boat etc. So in a sense, old worked out just as well. Next time if $$$ are available, maybe new.......
She drives me boat,
I drives me dinghy!