VoLTE could be launched solely on band 3 - my only reasoning was if they were to push ahead with VoLTE, then converting 900Mhz to 4G would be more beneficial (you may not be able to use data that well - even though 4G performs much better than 3G on weak signals - but you'll be able to hold a call, at least). That was my reasoning.
If they're not interested in opting for VoLTE (and they'll be stupid not to have that roadmap in the future), then yeah, 3G on 900 Mhz would make more sense.
Also, if they refarm, I wonder how/what combination they'll use the spectrum in. Warid's 4.8 isn't enough for 3G (but is sufficient for 4G), whilst their existing 900Mhz is sufficient for 2G, 3G and 4G. I also don't see them doing this everywhere because this might squeeze their entire 2G spectrum - unless they can integrate Warid's spectrum somehow for 2G.
Edit: formatting