Your core plan allowance is 6GB, and you have a promotional offer for 6 times this to get you to the 36GB a month.
You'll get this every month, but for the Virgin Mobile data rollover option that is only your 6GB core allowance. So you can potentially have up to 42GB in a month to use i.e. 36GB + 6GB.
As far I can recall, the rollover data gets used first, followed by promotional data, and then your core data allowance.
If you change your plan at any point you'd lose the 6 times promotional offer you have now, and would get whatever the available plan and promotion is at that point. If you don't change your plan you'll keep your current allowance offer unless you're advised differently by Virgin Media.
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I work for Virgin Media - but all opinions posted here are my own