Can anyone confirm that with the 2degrees prepay carryover plans, can you freely switch between plans each month, and keep any carryover data?

I presume this is the case (after trying to read website terms), but just wanted to confirm before proceeding to do this with a secondary device.

e.g. first month purchase $49 plan for 10 GB, and then the next 5 months use the $9 plan (250 MB), giving a total of 11490 Mb (11.22 GB) to use over 6 months at cost of $94
And every now and then purchase a one off $49 plan to increase data.

(disclaimer: looking at more flexible data options instead of 2D $99 for 12 gb / 6 month zone data plan, for mobile broadband at a bach, where usage changes every month, depending on time of the year)