Depending on how far you need to tune it, you should be fine with just adjusting the 2 bottom tuning rings. If you need 29Mhz, then you might have to cut it a little bit.
Be careful if cutting it, that fiberglass is very sharp, wear gloves and safety glasses.
I guess it just depends on how 'well' you want the thing to work on 10 meters. The rings deal with impedance matching, have nothing to do with resonance. Cutting the thing does deal with resonance for frequencies higher than the design frequency. That 'cutting' is also a "no way back" sort of solution, so don't do it if you ever intend to use the 'A99' on a lower band again. Those rings can only 'stretch' things so far, and adjusting the antenna's length will raise that 'stretch' range from what it used to be.