I'm fairly confident in saying no. Multi day tickets bought before any multi day tier pricing is implemented will not be subject to tier pricing as long as they are not expired.
The reason for my confidence: When tier pricing was introduced for single day tickets, any existing single day tickets without tier designation were still valid any day. And when Disney got rid of the Premium pass, any Premium voucher still valid was exchanged for the most equivalent pass rather than the one closest in price (so given a Sig+ instead of a Sig).
As long as a ticket media is valid, Disney is still going to honor it. Heck, Disney still takes entry tickets from decades ago in exchange for a current ticket despite different face values because they don't have expirations.