Synology RAID with different size HDD

[.Adam.]

I've had a DS212j for a few years with one 1tb HDD, and would like to add another and mirror it. I understand that you can use different size HDD, so was planning on adding a 2tb to it to mirror. I realise that all the time I have the 1tb drive in the system then I will only be able to use 1tb of the larger HDD.
I was planning that in the future when I have used the 1tb capacity I will swap the smaller drive for a larger one, but can I then use the full capacity of the 2tb that has been running at 1tb for a while, or would it need to be reformated?

[.Adam.]

That's great, thanks.