The memory sizes on CompactLogix and ControlLogix 1756-L6x controllers is fixed.
If you cannot reclaim enough memory by deleting unused tags shrinking unused arrays, you need to get a 1769-L35E controller.
Changing controller types is super-simple; just select the new controller model in Controller Properties. Because you're staying in the 1769 Ethernet controller family, there will be no changes to the I/O structure.