I don't see the forum categorization on Discourse as different from the one we have now, as it relates to that fluidity. In both cases there are separate forum categories for T1 and T2, but of course anyone can participate in either category, regardless of their personal type of diabetes. In Discourse it is also possible to write a post and not place it in ANY category, if it really belongs in several. Uncategorized posts there would more or less correlate to our current "General Diabetes and Pre-Diabetes" category. When in the forum, you will be able to look at posts within specific categories or all the latest posts, which will include anything that's not categorized.
I hope that allays your concern!