I am with you: it really is worth keeping the ROYAL evo/pro/SX. A really smart TX, still able to cope with most challanges a model might confront us with. Although I have other transmitters (as the ProfiTX, the mc 4000 and ...) I don't want to miss my ROYAL SXs (un upgraded Pro 7 and a Pro 12 (now SX 16!)).
So what follows now is just estimating: The upgrade of the software won't be that expensive and keep in mind that they will check the complete TX: The is no item that leaves the MULTIPLEX service unchecked! If there are no things to be replaced I would estimate cost of not more than 50 €. But the postage might be a bit expensive - for it has to be send to Italy. You will see as you have to pay the postage to the service. Forth and back I would estimate about 30 €.
The next point is the change of the HF-module. The module is no longer on the MULTIPLEX list (see homepage). But the transmitter is still in production and so should be the HFM-4 module. At the moment there is one offer on eBay for 85 € plus postage. So all in all the upgrade might easily sum up to about 150 €. The alternative a a secondhand one you might find on eBay: I have got two and in both cases didn't pay more than about 120 €.
Your basic problem is that the softare version of your TX ist to old and you can't avoid sending the TX to the service - so it is not your fault and neither that of MULTIPLEX, ist is just the technical development and progress.
Changing form xx-MHz to 2.4 GHz is worth the money - without question! The "new technolgy" (it has more a less become a standard over the last ten years) offers so many advantages: beginning with the faster response (realtime response), no freqency problems and not ending with the telemetry options. And by the way: who wants to live in a cave like stoneage people.