Window Treatment Rule Change

First, read your documents. You may see language in the bylaws that allow the board to enact additional rules as long as they don't contradict the bylaws or CCRs. Those may or not address window covering, but there may be something about preserving the original design and look of the community

Generally it's a good idea to poll homeowners on proposed rule changes - a good way to ensure compliance. In this case, you should ask The board what bought about The change. Are people using blankets, aluminum coupons cardboard, etc., instead of curtains? Usually the side of the curtains facing The outside are white - what's been showing up? Perhaps a number of owners have asked for uniformity because some people have stuff in their windows that is downright tacky (like my neighbor, who has as aluminum foil like something in his living room window!)

You mention previous boards had rules about window covering so this doesn't sound new. You might want to find some neighbors who feel as you do and then all of you can attend the next meeting, express yourselves and see what happens.

Grandfathering is not automatic.
It has to be specified in the rule.

That said, if the membership received approval for complying with the initial set of rules (or simply followed the rules without approval) some damages would be incurred for the rule change. In my opinion, the Association should pay for those damages (but that would likely take legal action).

An easier way is to gather owners together (perhaps via email) and present the issue to the board. A think a fair compromise would be:

Items under the old rule may still be used but when the item needs replaced OR when the property is sold (whichever is first) the item must be in compliance with existing rules.

Too keep this from happening in the future, I would also urge you to gather support and adopt a grandfather clause into your governing documents. That is one achievement I accomplished I am very proud of.