Only way of which I can think to do that is two separate data models in the application, one for the pre-current load and one for the current load so you would be able to do your comparisons, but I am not sure how the heck you do the swap, I guess on each load you would first need to load that prior QVD you created into the pre-current model and then do your pull from the DB and create new QVD etc... I am sure there are likely better options, but only one of which I can think.
I had another idea, but I got interrupted and it went right out of my head, and I cannot think of it again, sorry. Oh, I know what it was, maybe using a flag field of some sort to keep track of the historical versus current records, so when you reload the history files, you set a flag to true, but when you pull the records from the DB, you leave that flag false, that gives you a way in a single data model to keep things segregated I think. That is likely the best approach I think rather than trying to do two data models. Hopefully this gets you on a decent path forward.
You can do a search on 'flag' in the following link to have a look at some ideas on how this is used in some situations, not sure we have one that directly correlates to yours, but hopefully it may help further:
https://community.qlik.com/t5/Qlik-Design-Blog/bg-p/qlik-design-blog
Regards,
Brett
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