Short answer - your plan won’t work. This is why…
I had a similar experience between v12 and v13.
I paid for v13 in the September at a discount, but didn’t ‘activate’ (use the download code) it until October only to find that some ‘recent’ buyers of v13 had been granted a ‘free’ upgrade to v14 that had been released in early October.
I approached Steinberg who told me I wasn’t eligible because I had bought v13 at the discounted ‘sale’ price; the free upgrades were for those that had bought v13 after the sale but before V14 was released - A very narrow window of about 9 days if memory serves!
I was quite annoyed but actually, after thinking about it, I realised that it made sense … or at least business sense and Steinberg is a business.
UK PRICES USED IN NEXT SECTION
Why would they give away something that I would have to have paid approx £165 for (V12-V14) having only paid approx £50 for when buying the previous version (v13). They would be losing £115 !!! That is not a good business model!
A parallel - or other way of thinkng about it - is buying a new car.
When the current model of say, a Ford Fiesta, is coming to the end of it’s retail life, the dealers will sell of what stock they have cheaper than usual to maximise sales of that model before the new one is available. When the new model ‘all singing and dancing’ Ford Fiesta with lots of ‘new’ bells and whistles is released, I’m not sure its reasonable to expect the dealer to replace the previous model we had just bought with the new one for no additional cost?
So here is an alternate strategy for you to consider…
Buy Cubase 15 at the Sale price now (£50 until tomorrow, September 28th) … Wait until Spring 2027 (when Cubase 16 glitches and gremlins have been sorted out) for the Spring Sale (when Cubase 16.2 or something is avaiable) which is typically 30% off This means you will pay about £55 (70% of £85 typical upgrade price from latest previous version).
In total you will have paid £50 for Cubase 15 and then £55 for Cubase 16 = TOTAL £105
That is a saving of approx £60. And hopefully you will get a bug free transition too!
Of course these prices are approximations but the strategy should save some money.
And by Spring 2027 £60 could buy you a litre of diesel !!! 
Hope this helps you get the best deal ?