Upgrade deal with Go5G Go Plus

I switched to Go5G+ 24 months ago and my EIP is complete now. The plan includes upgrades every two years and they are currently offering $830 promo for trading in 16 Pro for 18 Pro (I know it was $900 last month). My question is if i should upgrade this year or wait for next year hoping that TMO will offer a similar deal (at least $800-ish like they do now for 15 Pro trade in)? Is it possible that TMO will scale down a promo next year? Do they guarantee some upgrade every two years until they retire? How much promo discount they offer for Magenta Max this year?