Hi guys

I've currently got just over 250k miles which by the end of the year this will be around the 300k mark

I am currently in the process of booking 4 return flights bhx - dxb for next May

So what is going to best to do, buy a saver ticket and chance upgrading when OLCI opens or buy flex tickets and upgrade immediately.
Or buy all the flights with miles?
Also if I buy the flex ticket can I upgrade at time of booking?

Thanks for your help