Sure you can use them, till they brake apart
You cant "upgrade" projectors with differnet bulbs. They are made as they are used, with more runtime they will get darker and at some point it make sense to get some new bulbs. Also its usefull to have a set of new bulbs in the storage to be quick if something happens.
The colors also will get worse with the time (dirt on the LCD Panels) this can be taken care of with cleaning the optic system. I advice to leave this cleaning to profesionall service tech ppl.
Also the colors can be out of convergence (these eiki are using 3 LCD Panels to get all the colors so when these arent lined up perfekt you may see something strange color behaviour)
is one adress to clean the system and get them into good condition again.
First do some testing. Set them up and view some testcards and different gray pattern to see if they are okay or if they are in a bad condition.
- White Picture to see the maximum brightness you can get
Gray steps for contrast
Color Bars for color
Small White Lines for convergence
Thes machines have a max resolution of 1024x768 pxl. (4/3 Ratio) when you blend them you can have some more in H or V direction.
Also its possible to Stack them to have more brightness in total.
There are many lenses for this typ of projector ( 0.8 - 8 Ratio i think) and these are available quite cheap because the lenses mostly life longer then the projectors
to play around and get into the world of projecting and mapping these are not so bad as someone may want to tell you.