Adapt an Intel Bay Trail Tablet to a newer version of Android

We have a number of 10" Android tablet PC's using Intel Bay Trail (Z3735G) Atom Processor with 1GB RAM, 16GB Storage that originally ran Android 5.0 for a Verizon 4G LTE configuration. This version is a bit old now and we would like to be able to upgrade them to a newer version of Android, like 7.0 Nougat or 8.0 Oreo without losing the functionality of the tablet to run the Verizon network. The OEM manufacturer does not know how or won't support us in the effort and we want to make these usable for newer software. Need the programmer to take the tablet and a newer Android version and verify that all hardware will support the newer software while maintaining full compatibility with the Verizon 4G LTE. Basically, we are not familiar with the Android software environment and this may be a very basic project but basically need to develop a new disk image which we can then replicate throughout the fleet of these tablets.