If your roof struggles to go up and drops like a stone, it may be worth doing.
If the above is accompanied by some residue or gunk on the chrome part of the original struts, then it probably needs to be done.
Testing the old struts
I weigh about 12 stone and I could fairly easily compress the old strut all the way. When I say easily, I'm talking about putting all my weight on it.
With a new strut, I got to about 3/4 compressed and my feet became decidedly light.
First step is to prop the roof up
Note V in top
Next, make a jig. The struts with the roof fully up are still compressed by a couple of cms and you need to support the bottom. Undo the bottom first
The jig should be such that it can slide along the roof. It requires a fair bit of strength to ease it along to the back of the van slowly once the strut bolt is lose.
Then do the top part. This, unlike the bottom, has a 12mm nut on it and you need this arrangement
You then have the strut off.
Putting the new one on is a reverse of the above but requires a fair bit of strength. The top bolt is easy but the bottom one has to be held firmly in place and aligned well with the threaded hole. This is, by far, the hardest bit. I did it on my own but two people may well be needed, one to hold the jig, the other to tighten the nut.
The other side requires a different, longer jig although I managed to do it by holding a shorter one in place rather than jamming it in place using some other part of the roof.
For the later Bongo, the roof strut dimensions needed are:-
988 mm fitting centre to fitting centre and piston rod length excluding fitting of 375 mm
Pleasingly, once done, the roof went straight up without the usual final push needed from inside the top before the beeping would stop.
