As you can see Conqueror and some Ace ranks will go Platinum I
The rest of the Ace will go to Platinum II
Crown I will go to Platinum III
Crown II and Crown III will go to Platinum IV
Crown IV and Crown V will go to Platinum V
Diamond I and Diamond II will go to Gold I
Diamond III, Diamond IV and Diamond V will go to Gold II
Platinum I, Platinum II and Platinum III will go to Gold III
Gold I, Platinum V and Platinum IV will go to Gold IV
Gold II, Gold III, Gold IV and Gold V will go to Gold V
Platinum I will go to Platinum I
Platinum II will go to Platinum II
Platinum III will go to Platinum III
Platinum IV will go to Platinum IV
Platinum V will go to Platinum V
Bronze I will go to Bronze I
Bronze II will go to Bronze II
Bronze III will go to Bronze III
Bronze IV will go to Bronze IV
Bronze V will go to Bronze V
We can see that a player that reaches Gold V will never go lower than that rank.
For more information check this picture bellow
Keep in mind that each server has his own rank, in PUBG mobile right now we have:
North American servers
Europe servers
Asia servers
South American servers
Middle East servers
KRJP servers
And each server has it own rank, so a player can be high rank in North America but low rank in Europe.