Best option for Email mailboxes migration from Small business server 2008 to Office 365

We have and Small business server 2008 standard FE (6.0, Build 6002). we want to move some specific mailboxes (15 users only - upto 50 GB of PST's) to office 365, we have exchange online plan 1.

PST files are higher in volume so it will take us ages to export, copy and import So we dont want to do this procedure.  

we dont wanna use the IMAP migration as it skips Calendars contacts etc. 

What we want is to Sync by some how SBS mailboxes with Office 365 and fetch all the emails, contact and calendar etc to office 365 without changing the MX record. When all the data is synced across to office 365 at last stage we want to change the MX record pointing to office 365.