When to upgrade kayaks?

So, I've had this poly kayak (Necky Manitou 13) for two seasons now. Am thinking about an upgrade .. to fiberglass or beyond. When's the best time to buy? Will I still have stability in a longer boat?

Dotty, so sorry I am not an American and can advise you

I have tried various boats at the belhaven kayak center in South, NJ. They really know their stuff. I traded in my 14 foot tarpon for an approach. I really wanted versatility and durability. If I am going to race, I will rent. The glass boats are beaucoup expensive. The longer and narrower the boat, the less stable. No way around it.

Upgrade, trade etc if you have the recourses.
A

GOT BOTH MY KAYAKS AT SPRING CREEK OUTFITTERS MT IRON MN NEAR THE BWCA -IT'S A HAUL BUT I SAVED LOTS-ASK FOR CHUCK-THEY HAVE ABOUT 300 KAYAKS ON HAND FACTORY SECONDS 1600. BOAT WILL BE ABOUT 1100. ALL TYPES-14FT CAROLINA ABOUT 900. 42 LBS PLASTIC NOT ROTOMILLED-

I replaced my Necky Manitou 13 with an advanced composite Necky Manitou 14. light at 42 lbs, just as stable as my 13. Great boat but expect to spend $2400