I would not worry about it as every 2 years is still relatively good maintenance.
Right now you are looking at 50% new coolant and 50% 1 year old which is great maintenance.
If you really want to be OCD, you could swap it out again in the spring or mid summer.
You could also do what I've done before which is unorthodox.
When I drive my car, and after it cools down, I suck out the reservoir and add fresh 50/50 distilled water and coolant and then put the radiator cap back on. Takes 5 minutes.
You should still change it the ole fashioned way
However, if you get behind in your maintenance your coolant will always be in good shape.
I change coolant out fully then use my topping off method which keeps the coolant in perfect shape. However if you only got 50% you can use same method and get about 75% fresh coolant in there without too big a cost. If you want to get to 100% its much tougher as the law of diminishing returns comes into play.
On my other Vettes, I change the coolant every year or 2 years at the most even the LS1 running Dexcool which is supposed be good for 5 years.