Lightroom not seeing files if folders are nested.

I have just moved from v10 to v11 on a new computer, in the process discovered that hundreds of files were not showing the previews which they had in v10. The reason is my fault, each folder of images contains a clients job and after a couple of years I decided to put all the shoots for a particular client in one folder to make it easier to navigate the drive.  When I imported them originally from the external drive into LC they  weren't in this one client folder, now LC doesn't see them so I presume I have disrupted the path.

My question is, can I relink them or do I have to either import them all again or remove them from their nested position in the one client folder?

As you may have quessed I use LC mainly for DAM rather than for editing(I shoot mainly on Phase One digital backs into their softwre), so please excuse my ignorance if this is a quick fix.