Yes, they will continue to operate. Of course you won't have the patches and other benefits of the new upgrade during that window of time.
I would recommend that you update all clients as soon as it is reasonable, but you don't lose their functionality when the core is upgraded.
Yes the older version agents will communicate with the latest version of core server without any problem. But until & unless you upgrade the client agents those machines will not be fixed from bugs.
Please take a look at the following link..
Do you mean that Security Scan will not work but Inventory Scan will work?
It is not in that way. Everything will work as it is..But you will not have the bugs fixed (refer the link above for the bugs and fixes), until you upgrade to SP3. Usually the nodes will not have the latest enhancements (if any) also until you upgrade all nodes to the latest SPs.
OK but the security update patches could be installed as usual, isn't it?
Is a reboot needed to install SP3 agent?
Thanks for your help.
Patch installations (e.g. MS Sec Updates) via vulscan.exe still work.
But I had to start the vulscan.exe twice in order to trigger some selfupdates. vulscan.exe and softmon.exe are out of date (new version on core).
After all updates had been done everything worked fine again.