Excel formula that "offsets" an amount to a later date / column

I am working on a project that is using a random number generator to calculate the period in which a customer will be "up-sold" (pay more each month). The customers are on a subscription contract so there is revenue every month.

My problem is that I want to add this Upsell amount one year from the random migration period that is generated and continue until the end of the model. As of now, the model works perfectly but the upsell amount is being added in as soon as the random period is met.

for reference:

cell BD6 is the randomly generated migration period cell DH4 is the number that is assigned to the date so the upsell can be added in starting that period. cell DG6 is simply the starting revenue Cell AU6 is the actual upsell amount in dollars that will be added in.

Hopefully i've provided enough information. I want to say the offset function will be used but I couldn't come up with anything so far.

This is the formula that is currently being used: =IF($BD6>DH$4,$DG6,$DG6+$AU6)

Thank you so much for the help]1