EXCLUSIVE

Billionaire heiress Petra Ecclestone has been on a mission -- to gut every inch of Candy Spelling's former mansion and scrub out its old-lady vibe. Well Petra ... MISSION ACCOMPLISHED. 

Petra bought the 57,000-square-foot estate back in 2011 for a cool $85 million ... but before moving in, she made it clear ... all the old lady decor -- antique chandeliers, dated fixtures, cherubs -- had to go.

After months of hardcore renovation -- we're talking 500 workers and millions of dollars -- Petra's redesign is complete ... and we gotta say, the primo pad is no longer where doilies go to die.

The Formula One heiress modernized every room, refurbished the master suite (adding a 7,000-square-foot closet), re-did all the floors ... and gave the place a cool, cocktail-lounge vibe.

And in case you forgot the "before" ... here you go ... but be forewarned, there's a lot of pink.