Just got to this thread, the issues are certainly somewhat confused, I will try to sort them out.
First, the 17g will be fine, and could be fine for days. I do this every time I re-set an aquarium. The fish go into a teemporary tank which is bound to be smaller, but if you can move over the heater, and the filter without cleaning it, and some of the decor/plants (just lay the plants on the surface, no need to "plant" them), thee fish will be fine for several days. I also start by siphoning out water from near the surface of the existing tank, sufficient to fill or almost fill the temporary, so the fish are going into the same water parameters. The temp should have a cover of some sort as fish will often ump (this is why I will never use buckets for this).
Second, the 47 liter tank is fine for the present fish. You do absolutely need sand, but you realize this and are changing it. The "plant" under-substrate is also detrimental to cories, but I needn't get into that. As for sand, you can use inert aquarium river sand (expensive but if it is river sand it is good for cories) in a dark colour (I would not recommend black with cories however), or you can use dark-hued play sand. Not all play sand is the same, but a quality brand is fine. You are in the UK and I believe others have mentioned Argos Play Sand, but UK memnbers can confirm.
When you get the tank re-set with the sand fill it with all fresh water, conditioned. There is no value in moving over any of the "old" water, and in fact it will be detrimental. Provided the parameters (GH, pH and temperature) are relatively the same, good to go.
The confusion over the cory species would not have occurred if people used the scientific name. Corydoras habrosus makes it clear to all of us which species.
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