If a modification is made to a volume submitter plan (even if minor) is it then considered individually designed? If so, it would seem the plan could be submitted for an individual determination letter. If it were not submitted for an individual DL, that would not necessarily mean you have a disqualified plan correct? It would just mean you could not rely on the volume letter.

You should look at Rev. Proc 2007-44. Section 19 includes a discussion on amendments to pre-approved plans. There will be other useful information scattered throughout.

If you amend the pre-approved language in a volume submitter document, you lose reliance on the opinion letter. But, if those changes are minor (as determined by the IRS), you can still submit for a determination letter using Form 5307. We have submitted amended VS documents in the past with what I would consider significant changes and were still allowed to use Form 5307.