Upgrading a 109 Adjustable or a 15X Balanced Adjustable to the latest 4th configuration (156) Balanced Adjustable requires a G250 annual service kit (dealer cost about $6.00) which will have the current poppet and balance chamber, and a new spring (part number 01 020 216, dealer cost $1.05)
In rare cases with a really old (late 60's) 109 second stage you may need a new lever (dealer cost about $5.00) as the tabs on the older ones are too large to fit the newer poppets. But you'll need a new lever anyway to install the current G200 dura poppet in your 109. And if you have to shell out $5.00 for a new lever, you may as well spend another $1.05 and upgrade the reg to BA status.
I have seen dealers charge up to $75.00 to update a 109. But don't let your local SP dealer stick you with an "expensive" upgrade, particularly if it is done as part of a normal annual service, as even if they gouge you with a 100% markup on the parts, $2.00 to 12.00 should cover any costs over and above a regular annual service.