I have 12500 fun tokens and holding period in 110 days
When I buy 37500 fun tokens , I reach a total of 50000. In this case, does the my premium benefits that it provides 50000 fun tokens continue from the 110th day or does it start from the first day of 50000 fun tokens premium benefits.
I believe your premium benefits will have 110 days recorded at the 12500 token tier, and 0 days recorded at the 50000 token tier. You would not be able to freely "age" your new investment, since that could be subject to a lot of abuse - anyone could lock 2500 tokens for a year, and then purchase 497500 tokens to immediately the top perks without the time.
Based on how the website is laid out, I would guess that you will be getting the best perk possible from your holdings (for example 0.05% cashback from 12.5k tokens at 90 days instead of 0%, and also 7 WOF spins from the 50k 0 day tier) instead of downgrading some of your existing perks though I'm not 100% sure it'll be the case. Perhaps TheQuin can clarify on this.