Debbie Rowe ENGAGED -- I've Changed My Mind ... I'm Marrying Marc Schaffel

Debbie Rowe has had a change of heart and decided to marry long-time friend Marc Schaffel ... and the flip flop has to do with her health.

Rowe -- who was famously married to Michael Jackson and had 2 kids while she was married to him -- tells TMZ ... when Schaffel gave her what looked like an engagement ring 3 weeks ago, she made it clear she would NOT marry him ... largely because she feared she had terminal cancer.

Rowe says she got the word from her doctors Wednesday ... she has sarcoidosis -- an auto-immune disease which is NOT cancer and not life-threatening.

She has decided to now say yes to Schaffel's proposal, telling TMZ, "He loves me, knows my kids, loves my kids and we'll see where this goes." Rowe says they have not set a date.