Why not ask Bramber how confident they are about the fit of a new trolly to your present base, your find out then if your desired way to replace your units will work.
I have found that if they say it will fit, it does ! Just make sure your both talking about the same type of trailer.
How often do you use your base, twice a year?
Another way could be to buy the trolly of your dreams and use it on your base with any extra support needed for that short trip. Then later get your new base that fits the trolly.
The cost of a full combi with delivery, will be cheaper than the two parts bought differing times!!