Tread lightly here, if you do it wrong, you could end up removing the profile from computers. 

Essentially, you want to replace the scope of the profile with a new scope that encompasses all the existing computers. You coudl try creating a new smart group that is a duplicate of the existing smart group and add a "enrolled before" criteria and set the cut off date. 

Then compare the contents of the new smart group and the old smart group and make sure you still have all the computer you expect. Then, if the groups match, change the scope of the config profile. Make sure so to only apply the profile all new computers, otherwise all the existing comptures will have the profile removed and re-added. 

- Try creating a smart group on devices that currently have the config profile installed.

- Scope this group to the configuration profile

I've used this approach when moving to a new wireless profile but without disrupting the devices out in the wild. As Tribruin answered, my newly enrolled devices will get the newer profile which I scope on a "Enrolled after this date" type group.

Thanks Tribuin and Bol.  Yes Im making the post because I noticed before profiles disappearing before - was nothing serious - but it certainly got me thinking. 

Your posts make complete sense and gives me a sense of direction now.  Thanks so much for your prompt help.