These numbers refer to Hotfixes and Patches for this current version.
You can upgrade ePO agent from 4.6 to 4.8 from ePO console. Try to do it on some machines and based on success rate do it on remaining machines.
If you are going to do it locally than I'll suggest to try with command /install=agent /forceinstall
I wouldn't suggest using the /forceinstall switch unless you have issues with specific clients. This will change the agent guid of the client and can cause issues - recreating new entries in ePO and esp if you have encrypted machines.
Well I have tried many times in my some critical servers where it didn't loose the policy as ePO understand Machines based on MAC addresses so I don't think so for issues.
But I'll suggest to do a test before doing it in bulk.
You shouldn't need to do a force install ever- unless there is issues with an agent- or you met the criteria stated in the KB. A version upgrade from 4.6 to 4.8 should work without issue.
This is correct, and should be stressed
The force options are really intended for troubleshooting, and if you don't understand the implications then you can cause yourself more problems than you solve. Please don't use the force options unless you need them.