I'm not querying you taking over all the assets incl. existing contracts, but you switching the account holder details on the contract w/out 1st getting the permission of that a/c holder.
Example: A parent rents phone line from YourCalls. One of his children places PN BB on that phone line with parent's permission.
Now that you are taking over YourCalls, you will takeover the parent's phone contract. You are saying that you will merge the parent's phone a/c into the child's BB a/c. Does the parent want that? Have you sought his agreement? It's his decision.
One side effect will be that the child will be billed for both phone + BB. Or will it be the other way round? It is quite likely that the patent wants to pay for the phone but allowed the child to have BB on it on condition that the child paid for the BB.
OTOH it may be that if the child is paying extra for having PN BB Only then the household will save money by having them together.
But in the final analysis it is up to the parent to decide how he wants it and you need to ask his permission whether he wants the 2 a/c's merged and, if so, how.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC