In general: Yes, this happens when maintenance work take place but the schedules for the period are not confirmed yet. Happened to me a few week ago when I booked an advance ticket and the planner said "train likely to be cancelled. Check again prior to departure". When I did a few days prior to the trip, one of the trains involved was cancelled and my ticket was endorsed for another train in the ticket office.
I guess in your case track work by SNCF might be involved. From the delay of about an hour I would guess that the trains could be diverted via the northern route (Mannheim->Kaiserslautern->Saarbrücken) instead of the regular southern route via Strasbourg.
From my experiences on the past: The delay estimate is ok. Nevertheless, in such a case I would try to buy the ticket from one operator instead of mixing DB/SNCF/Eurostar tickets as this might save you some discussions when the final schedule is confirmed.
Unfortunately, on your case this will leave you with SNCF only as DB does not sell tickets from Paris to London and you want to go via Paris. SNCF tickets for such an international services are usually available 6 month in advance - in contrast to domestic services which usually go on sale only 3 month in advance (as SNCF toys a lot with booking windows, best search for a ticket for your desired date and register an email alert, if the ticket are not on sale yet).
Alternatively, book a DB London ticket via Brussels (booking tip: put in Brussels North as intermediate stop as this will omit the Thalys trains). As this is one ticket, you stand a better chance when the schedules change or a delay happens. From Stuttgart this takes longer than the trip via Paris, but when one hour is added to the Paris route, the difference is not that big any more. Connection from ICE to EST in Brussels is pretty relaxed as there is usually about 1,5h time between ICE arrival and EST departure (and checkin in Brussels is less chaotic than in Paris - at I least I try to avoid to change to EST in Paris). These tickets can be bought 6 month in advance.
hth
Charly