I'd recommend you read this post from last year about several people's experinces with ePMP 2.4ghz gear. We also have changed many 900Mhz deployments to 2.4Ghz.
http://community.cambiumnetworks.com/t5/ePMP-2000-and-1000/Does-2-4GHz-makes-sense/td-p/51783
Yes, all things being equal, lower frequencies (900Mhz) should penetrate trees better. But the ePMP 2.4 ghz gear can output higher power, can user much higher gain antennas, and most importantly - can get MUCH higher troughput at the same signals.
As Chris has said, if you're currently getting a -70 with FSK gear, that's likely with what... an 11 dBi yagi on the SM maybe? And you'll get 3.3 Mbit of thoughput under perfect conditions, shared among all subscrbers. With ePMP @ 2.4GHz, we will mate a connectorized SM with a 22 dBi Dual Pol Grid, and the AP/SM will often connect at that same -70 or so. So what you're losing in propogation, you're gaining in real gain. And again, most importantly - usable thoughput - that AP can have 20 or 30 or 60 Mbit of usable throughput, instead of 3 Mbit. :)
Plus, you can further help a low signal @ 2.4Ghz by narrowing your channel width sometimes to. Again, your mileage may vary, but if you go down to a 10 Mhz wide channel, that can increase gain by a couple db in some cases. You will lose a bit on the maximum throughput capability, but may gain some back on the 'gain' in the system. Likewise, sometimes even going down to a 5Mhz channel can again increase the final SNR, even at the expense of halving the theoretical throughput. But, if you have a low user count sector on FSK right now, then absolutely it's very very possible to replace that with a 2.4Ghz ePMP and even at 5Mhz widths (similar spectrum width), and to often shoot through just as many trees, and get similar signals, and to increase bandwidth by 5 times.
For us, we've kinda standardized on 10Mhz channel widths in 2.4Ghz and it's working really really really well for all but the worst conditions. At the very least, we've been able to move 75% of our 900Mhz FSK people over to ePMP 2.4, and those people are MUCH happier. And, the remaining 25% who are stuck on 900Mhz... those people now don't have to share the 3.3 Mbit agregate throughut with all those people anymore, so their internet works better, even without being abel to upgrade them. :)
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