If you void the warranty its the cellular providers warranty you void. Ie. That 7 day return policy is provided by each network, not the manufacturers. So, swapping a sim to another network can mean:
1. The 7-day swap out falls away (if they could prove it)
2. The 2 year warranty falls away - afaik most phones actually come with a 1 year warranty and it is the networks who offer the 2nd year.
I was told that if I switched to another network the warranty would decrease from 2 years (if I used a voda sim) to 1 year. That implies that Huawei offers a one year warranty and that vodacom offers an extra years warranty after that.
Again though, i don't know if they can see whether or not the phone was used on another network.
I suppose sticking in the voda sim once a month, if that will work, isn't a major hassle, but if I only have to do it before the phone needs to be sent in for repair, then that would be easier (first world problem).
On a side note, do you have to put airtime on the card or make use of it (calls, data, sms) to keep the number, or do you automatically keep it for life once RICA'ed?