I currently have bit dilemma. I current have the Planet Ocean 8500 in 45.5mm with the orange numerals. But really like the look of the the 2019 Seamaster diver 300. Would getting the diver 300 be a downgrade from the the Planet Ocean I currently have?

 

They are very similar but very different watches.
To make your choice harder....

The only 3 major "downgrades" I see from going PO to SM are:
1) You lose the convenience of the folding clasp
2) You lose the jumping hour hand feature
3) Shorter power reserve

3 "upgrades":
1) Way better bezel action feel
2) Thinner = more comfortable
3) 5-year warranty