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You do not have to modify anything, it fits perfectly and it compliments the D&C system I have and love it, It gives you 3 levels

Here is a post Bobby put up on FB last week and he did say it would be available as an accessory for older models

Mcmaster-Carr has a wide selection of stainless steel springs, you might try there https://www.mcmaster.com/springs

You must of removed the Dome to change the gasket otherwise nothing should of changed when installing the new gasket, specially when the new one is thicker

Thats me too, I got my 1st KJBJ 6 years ago and have never had a problem controlling temps and I have done countless over night cooks with zero issues

It should be pretty tight but removable, if it spins easily you could add more gasket materiel to tighten https://www.amazon.com/onlyfire-Gasket-Replacement-Adhesive-Classic/dp/B06XR3XHZH/ref=sr_1_15?ie=UTF8&qid=1544810249&sr=8-15&keywords=kamado+joe+gasket

If you have a place to hang and wire it this motion LED light is awesome https://www.amazon.com/gp/product/B01MUBBGAS/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

As long as it meets these dimensions it should

I really like Blue's Hog sauce but I do agree its a little thick and sweet, I thin it out with apple cider vinegar and its my go to for Chicken & Ribs, I never tried it on Pulled pork

Thats really weird, Does the SS top fit inside

My Classic and Big Joe are also 5 years old and both have the new firebox, maybe they sent you one for the Big Joe, Does it have 6 sections or 5?

Personally I like the Kamado Joe C.I griddle because it cleans so easy, You just scrape it, wipe it and spray with oil and put it away for the next time you need it, Kamado Joe also has a Soap stone that is even easier to maintain but it is fragile.

I second what fbov said

You can do a lot with 2 levels but if you really need three, Ceramic Grill store sales a rig that allows for 3 level cooking on the Classic & Big Joe