In an STM32 project I am using the 7.5.3 so far with good satisfaction. However, I am also aware that there are many versions after this release. I wonder, from STM32 point of view, is there any newer version that is recommended for an upgrade?
I would always recommend using the latest version. Even if the new features are not of interest to you (http://www.FreeRTOS.org/History.txt), the additional diagnostic capabilities may be of use and save you some time.
The latest version is V8.2.1 - in theory you should be able to just drop the latest source files on top of the files you already have. Note there is a new header file (deprecated_definitions.h) you will need, but if you copy the files from the latest distribution that file should be copied too.