Renewal with a change of plan

I am trying to work out the best way to handle this scenario - we have different cost for membership  for adult s(21 yrs), youth (8-20) and child (0-7)  At year end  7 year old needs to renew. I need to have them now sign up for a youth plan.  Likewise when the 20 year old needs to sign up for an adult plan.  I thought that might be considered an upgrade - change of plan- but that seems to be something different.,  We would like to retain the date that they first joined (the from_date I think)  But if they change plans it won't be a renewal any longer so we would lose that information.  Also, we would need, somehow to prevent them from joining through the child plan/youth plan when they are too old for them.  Is there anything in the system that could be used to handle this, or perhaps there is code that could be interjected to control this.  If you could point me in the right direction to insert some php or js custom code, perhaps I can work something out there.  Any suggestions most welcome.

I am looking at changing to a single plan which will work better for this case - but I have to ask the user what age group they are in and then I can set the price based on the radio box 'what is your age group' . I seems pretty silly to ask that question and then ask your dateofbirth - which we need - and then ask if you want to purchase a adult, youth or child season pass (and set that price accordingly). - This works quite well except for 2 things - 1. no way to validate that they are choosing the right season pass for their age and 2. the questions we will get from the members - 'why do you have to ask us 3 times what age group we are in - dob, what age group and what season pass?'. In our earlier site we simply asked the birthdate and , through a bit of js , worked out everything else. Is there anyway to insert some custom code for this? Other suggestions as to how this can be handled?