ESA / Tax Credits migration to UC

I received the migration notice letter a few weeks ago saying that my wife and myself need to apply for UC. I am on ESA and was receiving child tax credits and my wife was also named on the child tax credits, so she's also received a letter saying that both of us need to apply for UC. We are no longer entitled to child tax credits as our 19 year old son is no longer in full time education. So being as it was the child tax credits that triggered the migration letter, will I still have to migrate to UC at this point in time being as it is now just income related ESA and not tax credits that I am in receipt of?
I know I can not just ignore the letter or my ESA will stop ,but was wondering whether I will still have to move over to UC now or not being as if you are just in receipt of income related ESA at this point in time you are not being forced to migrate to UC?

Hi Gary,
Thank you for your reply. I have had the migration notice which states that I must claim by 22nd November so I don't think it's a case of seeking advice as to when is the best time for me to apply for UC. I am within that group of people that is /was on tax credits and income related esa which are being sent the migration notices ,but I am no longer eligable for tax credits. So is it worth me ringing up the DWP to ask whether I still have to migrate now or at a later date ?
I'm pretty sure they will say that I still need to migrate as at some point everyone will have to , but I would rather it be at a later date.