5G is not magic, it does nothing to improve the Mbps per MHz over LTE - which remember is the same for 150 Mbps LTE as it is for 1.2 Gbps LTE. LTE's current max is 256 QAM, but the latest 3GPP release adds 1024 QAM support to LTE. That's a 25% improvement in Mbps per MHz, though you don't quite get that much in the real world due to error correction, and it will obviously only work in areas where you have maximum bars today. That's also going to be the max for 5G. The only difference between 150 Mbps LTE and 1.2 Gbps LTE today is a single client getting to use a LOT more MHz at once, which is why people arguing over the difference in phones that support "faster" LTE are wasting their breath, since you don't need a gigabit to a phone and can't get it in busy areas where a single client won't be able to grab that much spectrum anyway.
5G attains its insane claimed future speeds by using LOTS of bandwidth, which will be opened up mostly at the very high frequencies in the 28-39 GHz range. Using 600 and 700 MHz bands for 5G will provide no bandwidth increase over what can be done with LTE. I'm not sure there is much of a business case for upgrading existing LTE bands to 5G since the major improvement of 5G will be in latency. 5G will be mostly greenfield spectrum deployments.
Upgrading 600/700 MHz bands from LTE to 5G doesn't make much sense, and even if it is done will not be useful for fixed broadband deployments at longer distances. So no, not a "brilliant" move at all (at least not in terms of broadband deployments with 5G, having lots of low end spectrum is worth just as much with LTE so there's no need for 5G) For phones 5G is little better - unless you are doing something that's latency sensitive like online gaming the difference isn't worth caring about.