RRS 87 is very clear about this:

87 CHANGES TO CLASS RULES
The notice of race or sailing instructions may change a class rule
only when the class rules permit the change, or when written
permission of the class association for the change is displayed on the
official notice board.

Some thoughts:

You can't just change anything that you want without approval of the class (either within the rule, or by requesting the change for your event).

You really should do this in the NOR so that people know what to expect, as it may impact their decision to attend.

Remember that for handicap races, the handicap system rules rank as class rules per the RRS definition of Rule.

Much of what Carl has listed probably aren't (and certainly shouldn't be) class rules so much as policies for class-sanctioned events (typically points regattas, nationals, etc.).

I have seen some OAs waive safety rules, which is a horrible practice.

Advertising is covered by separate rules (RRS and World Sailing regulations), so there are limits on what can/should be altered in SIs.

A common work-around for mixed, non-sanctioned events is to state that there shall be no penalty for breaking rule (fill in blank). Technically, this isn't a class rule change, you're just making it known that it won't be enforced. Again, this isn't always a good idea.

Hope this helps.

Mike