I already have the latest version of Igrid 5 what should persuade me to upgrade to Igrid 6.
Is there a comparison chart that shows the main differences or additional functionality?

Thanks.

It's hard to decide whether you'll get some benefits in your particular app(s), but maybe, the comprehensive What's New document will convince you:

http://10tec.com/FS/Prod...Whats_New_in_Release.pdf 

As for me, I would draw your attention to the fixed bugs, some improvements like auto-scrolling when dragging column headers and better Windows 10 support.

I have an MS Access application that uses iGrid 5.0.0.52 extensively, if I upgrade to 6.0.32 will I need to recode all my forms, or are the methods and properties allow for an almost seamless transition?

Most likely, the transition will be "almost" seamless, but this depends on the iGrid members you use in your code. You can open the aforementioned What's New document and find all points marked with the [Code-Upgrade] tag to see whether the changes in iGrid v6 affect your code.