Making a backup is always the preferred route. If it is me - - → I would buy an external USB, do the backup, and then store the backup in different house (safe deposit box, parent’s house, office desk).
HOWEVER, it is theoretically possible to do it without a backup;
Method 1: BRUTE FORCE:
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remove the 2x3tb drives;
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Connect to the drives using a PC (note: The drives are in a LINUX format, so some additional software is required)
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install new 6TB drives and let the EX2 do it’s thing.
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When complete; you can copy data from PC to new drives
Method 2: Trust the software
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In the EX2 dashboard menus, there is a place to change RAID configuration. One of the options is “expand”. If you hit this, The system will prompt you to replace first one drive, then later (after it builds all the data), it will prompt you to replace the other drive.
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I suspect (cannot confirm) that it is not critical to chose “expand” before starting drive replacement. However, if you just swap one drive, then rebuild, then replace other drive. . .it sounds logical that you would wind up with 3TB capacity (not 6TB). HOWEVER, if you expand. . it will then claim the extra space.
There are two threads that may have useful information:
If I was doing it. . . I would make a backup, then “trust the software” 
