Yes this is very common. Especially now that most of the new stuff is starting to relay on performance based navigation. It makes perfect sense to get a 747-200 upgraded if the plane still has life.
Take Connie, their 747-100/200s have the Canadian Marconi FMS and EFIS. They now have a new lease on life. I am not sure but those planes I think now have RNAV(GPS/GNSS) approach approval. They also meet all the requirements of AC
90-100. IIRC the Marconi has tha ability to accept a lot of the requirements for RNAV(RNP) approaches as well (AC 90-101) so those new approaches are the wave of the future as well. They also have MNPS and RNP enroute which allows less separation and stuff.
All this stuff means the planes can go on and on for years to come. I think those FMS' in the 747-100/200s are set up to accept new things in the future, that is called FANS(Future Air Navigation System). Needless to say, upgrades like that also raise the value of the planes. I mean a 747-200F with a FMS like the Marconi is more valuable then a 747-200F with the old INS system.
It makes perfect sense if airline plan to keep older airplanes around. Sooner or later those un-upgradeded planes flat out won't be able to fly since the old things they used..... VOR/NDB etc etc are so few and far between that they can't support that kind of navigation anymore.
I think the NW
DC-10s were modified just right for the newer requirements for flying NAT
TRACKs back when MNPS, SLOP, and RNP4 and 10 separation and all that started to be implemented. Since the planes where used a lot on TATL flights, they needed to be upgraded to stay able to fly those routes.
Overall, all these upgrades make flying much safer which is the best thing over all!