It is not easy and fairly expensive as I asked the same question. I think it is something to do with adjustments that are required to the axles. You would need to check with an HGV mechanic as I can't remember the ins and outs.
Yes it can be done - cost approx 2.5K dependent on the chassis.
All the tyres need to be changed to 10 tonne slightly larger ones and also the the springs will need to be changed. New tacho fitted due to the change in tyre rotation - and the vehicle will have to be presented to the ministry for inspection and approval for new plating cert. Ring up your nearest chassis suppliers ie MAN, Iveco - dependent on which chassis you are uprating and ask for service dept- they will give you a rough guide to costings.
does'nt sound good, think she is gonna end up stuck with it, it also 25yrs old too which makes not worth a lot anyway! She wants 4k for it with no MOT on it!!!