Raid 5 storage Increase

I want to increase the size of a Raid 5 array from 3x 300Gb disk to 4x 1Tb disks.  Plan was to swap one disk at a time till they are all 1Tb, will this work, will this leave me with more storage. Or is there a better way of doing it.
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It depends on the RAID controller that you have.
Probabily you can only use the 300 GB partition on the 4 disks; but it depends on how old and which model is your controller
In RAID 5 you can only use the amount of storage on any disk equal to the smallest disk in the array. If you put in a singe 1TB disk you can only use 300GB of it. Also, each time you swap one disk you will be forcing the RAID array to be rebuilt.

The simplest thing to do is to bakup the current array, then remove it and delete the 3 300GB disks. Then swap them for the 1TB disks, configure the new array, and restore the data.

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What carlmd says is generally true, but if you have a  suitable  controller, you can change one drive at a time, waiting the time for rebuild every time.
Then you can reconfigure the raid and expand its capacity with a functionality named Online Capacity Expansion (OCE).
See here an example

Obviuosly having a former backup is a must.
This technique puts your data at risk.  Don't do it.    (Plus, even if you did do this, you would still end up with a single LUN that is 600GB).  Do it the safe and correct way.  Backup, build the new RAID, restore using a product that will build the file system the way you want to.

Also, FYI, you can't boot a LUN > 2.09TB (assuming windows and this isn't an itanium)
As above - depends on the controller as to whether you can use the extra space or not and it's a long time to be without parity since each rebuild may take a day. Any unrecoverable read error will mean it won't rebuild so you have to restore from backup.

Booting from a LUN greater than 2TiB (GPT) only needs EFI / UEFI support, not necessarily Itanium nowadays.
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