This one comes up in the TV boards occasionally, but I'd be 95% confident the answer is the same.... which is no.
Depending what model the old router is (you've not said), if VM wanted it back then you've done well to not have a non-return charge imposed. VM will also have associated the new kit they're supplying, with your new account.
Even if they wanted to alter the equipment details in this way (and I'm not sure they would), it's probably asking for a whole world of pain from the C/S systems.
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