Hello,
You can upgrade the firmware version to 2.9.1200 without any worries, it is even recommended to upgrade the firmware to the latest.
The latest firmware version is not build-in in the driver because MNPH29C-XTR is an EOL (end of life) card and also reached to its EOS (end of support) period and is not maintained anymore by MLNX_OFED driver but there should not be any problem to upgrade the firmware separately from the driver.
Best Regards,
Viki