The Good News -- The Women's Tennis Association (WTA) has announced massive rule changes allowing athletes like Serena to wear full catsuit outfits during 2019 matches.

The Bad News -- The French Open -- where the whole catsuit drama went down -- could still ban it because the rule change doesn't apply to Grand Slams.

Remember, French Open officials created a huge controversy after they told Serena she couldn't wear her custom-made Nike compression suit ... despite the fact that it was designed to increase blood circulation in the wake of her health issues surrounding blood clots.

The ensuing outcry was LOUD -- with everyone from ex-pro tennis player Anne White to ex-Iran president Mahmoud Ahmadinejad racing to defend Williams.

In an effort to prevent similar drama from happening ... the WTA announced in rule changes this week, "Leggings and mid-thigh-length compression shorts may be worn with or without a skirt, shorts or dress."

Translation ... catsuit's back!!!

The problem -- the rule changes only apply to WTA Tournaments ... NOT Grand Slams.