What do you think about this solution?
In my case, after a transaction from segwit addresses I ended up with my entire balance on one legacy address.
So, what if before each spending, I check the balance of the wallet (in my case only one address) and calculate the change amount before payment is made.
So, for example: if my balance is 0.55 and I need to send 0.25 to somebody, the change will be 0.30 (minus fee).
Now, I can easily include in sendmany a new (or existing) segwit address I generate and make sure the fee is deducted only from the new segwit address. This way, I will end up with 0.30 minus the sending fee in a new segwit address. And next time, when I send out funds, I always do this, so I will always end up with my balance on one segwit address.

Is there anything wrong with my assumption?