since whatsapp doesnt support it then the best way is to use 3rd-party apps/software
but they never is free, mostly are paid ones
examples are mobiletrans and ifonecare
i tried these but doesnt really works, basically it just installs an older version of whatsapp that backs up the chats to internal memory and initial it to transfer to the iphone
and using the iphone it will be packed as an 'ios app backup' to restore the backup chat history
after one week of trying everything (for free and also one paid app, wussaper) i just let it go
those past chats are gone now hahaha

this one i think i have done better by forwarding it to my 2nd whatsapp (using whatsapp business) account but this is very messy indeed