Hi Steve,
Unfortunately - there is no 'magically transforming' to 836A - this is not possible!
The 'A' variants - have additional IQ circuitry which is missing in the non 'A' models.
With regard to 'enhancing' the 815 to 830 or 836 - this is work-in-progress and depends on many factors
I am currently Reviewing the 815 - thanks to the generosity of my Test Equipment distributor (I will BUY a device - either the Rigol or equivalent Siglent)- and been 'posting' my mini findings as I go along (another one scheduled later today) - and then a will publish a more complete summary.
My specific 815 thread is here ...
https://www.eevblog.com/forum/testgear/rigol-dsg815-testing-feedback-and-comments/msg3122450/#msg3122450The discussion on 'enhancing' 800 series thread is here ...
https://www.eevblog.com/forum/testgear/upcoming-rigol-dsg815830/25/Despite the possible enhancements - the DSG815 is a fine instrument - although its a little dated design (back to 2015) - it performs extremely well.
If you can live with the 1.5GHz limit - then its really good value - I like the compact size.
Take Care & Good Luck