Yes! This is commonly done. Here is how it works.
Find the direct equivalent to your old cartridge in the current range, note down the price and the available trade in discount amount.
You can then deduct that amount from the price of any cartridge higher in the range.
An example will make it clearer:
Say you have a 25 year old 20B2 cartridge that the cat has mangled. The current equivalent model is a DV-10X5 mkII, the price in Dec 2018 is $945.00 with a 20% discount.
20% of $945 is $189.00. You may deduct $189.00 from any other cartridge in the range above the DV-10X5 mkII model.