I went through this process about 18 months ago, did I want to retain my 'grandfather' rights. After discussion with my family the answer was a resounding yes.
Before I give my reasoning I first must question your weights.
Without the 'rights' the max train weight is indeed 3500kg however the weight of the Cali is the unladen weight which includes a fuel load (90% I believe) and a driver (I think 75kg). The difference between that and the train weight is the weight that you can put into the Cali and weight of what you can tow including the weight of the trailer. I tow a light weight caravan which just keeps me under or around this figure but not a problem because I have the 'rights'.
Why did I go to retaining my rights. Not because I occasionally tow a caravan but because we may want to either go-over to the dark side and get a motor home (I can hear the sharp intakes of breath even before I post, such heresy) or hire one for a tour of Australia/New Zealand. My wife was coming up for early retirement and we didn't know how or in which direction our lives would take and the decision was, for the sake of a bit of hassle, keep the rights which leaves your options open for another three years when you can revisit the issue. You will be better informed as to your needs with three more years of experience/health as to whether to retain those rights.
Hope my two-pennyworth helps.