If you plan to do this for least amount of extra out of pocket then you need to re think your approach as it is antagonistic to your goal
when you trade in, you are essentially discounting the sale of your instrument
I don't even know why they call it trading in (trading usually means you are getting something of relative equal value in return, this is not the case in the transaction you are looking at), it is not a trade, it is accepting a low ball offer. you are lucky to get a whole sale price within 10 percent of what they can buy it directly from their distributor for.
Find out in your region or markets what they sell new for by price checking various dealers. Find out the are going for in the private individual to individual market.
then get quotes on wholesaling it to a dealer (note the quote should be independent of any purchase otherwise they pad the trade in value because they make it back by discounting less on the purchase they plan on your making).
Then see how far away you are from what you budgeted for the upgrade.
generally you will get more for your piano by putting in the work to list market and negotiate pricing on your piano to another individual .
you will also get more piano for the money by buying used from someone else (just the like the person buying your piano gets a better deal than a similar instrument from a dealer, since even if you're is 'new' it's not retail new, so don't expect to get retail for it, you don't have overhead to recoup, you can likely ask for the high side or high end plus 15% of a private sale used average so long as it's still less than the low end of retail new)
what business sense does it make for a dealer or retailer or re-seller to pay to you more than they can get from their normal distributor? The store has overhead to make up such as lease utilities insurance payroll commissions taxes and profit etc
I believe some of the best uprights made that are surprisingly affordable relative to the quality ( pretty much "heirloom" quality build) are Charles Walter pianos, sometimes you can find really nice MH uprights at a decent price too. The Charles Walter grands are expensive (but still better values vs similar condition Masons or American Steinways) but worth it and likely outside your budget
but you would be surprised what you can find on the used upright side of things.
https://www.chicagopianos.com/052311_update/charles-walter.htmlyou might email these kids and see what they want for this, i think new these are about the cost of an decent quality Asian or even used premium Japanese Grand, so used you might be surprised how much of a deal they are
on the little bit small instrument
i have seen very nice /clean pre owned CW at dealer/retail in the under 4 thousand dollar range, at the 3500-4500 range, they are super attractive
https://pianomarketplus.com/pre-owned-pianos/#Charles R Walter