Chaos at start if it’s a sudden change. Groups that been persecuted for years will want some blood letting as revenge. So you can have a civil war on your hands. This can attract the more extreme groups from either side into it. Froeign groups of extremists can be more violent in their methods, suicide bombing packed mosques etc. Especially if some groups just want it to turn into massive civil war. This could draw western countries in, which means more important targets for the extremists. Certain Countries could finance those groups to help tear Iran apart. Look at foreign fighter groups in Iraq. Were fighting against US, however started strict Sharia law in their areas. Executing Iraqis for various reasons. This lead to some Iraqi groups offering deal to US military, certain Iraqi groups would cease actions against US troops, in exchange US helped them against these Foreign fighters. Reports of this happening in parts of Baghdad.

You’ll have US and Russia financing their aligned groups in the country. Both of those have a interest in the direction Iran takes. Plus both will have their spec ops in country. Isarel May see it as a free for all in Iran going after who they want. Iraq and Saudi will want their say as well.

Or it may all go smoothly and no dramas at all:)