I do not believe there is any stipulated notice period for informing a client of a take over.
DMP's and their providers are very loosely regulated compared to IVA's and, when I was in one I wasn't informed of a change of firm until well after the event.
These things are often commercially sensitive and are kept under wraps until the deed is done, at least.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014