Both Windows 10 and 8.1 on a PC. Is a choice necessary before End July

My brother in law downloaded W10 some time ago and reverted to 8.1 having had a problem with some files . He has just asked me the following question:
As you know I currently have Windows 8.1 installed on my PC.
Some time ago I downloaded the free issue of Windows 10 and installed it on my PC. I found it difficult to find all the existing information that was previously saved on my 8.1 version so reverted back to 8.1. The update to Windows 10 is still in my Windows update folder. After 29th July will I still be able to re-install Windows 10 even though the free update offer will have been closed?

I understand he is getting W10 notifications or updates so Microsoft are seemingly think he is a W10 user. Does he have to decide to switch to W10 and would he lose W 8.1 or can he keep both versions on the one machine?
Any advice greatly appreciated
Dagwood

If he has installed W10 on his machine before 29 July then the machine will have the digital entitlement recorded for activation of W10. It is my understanding that his machine will then automatically activate for any future installations of W10 even after 29 July. The only thing that could impact this is if he changes the hardware such as the motherboard, he would then lose that digital entitlement and the automatic activation would fail. ( A phone call might sort it out but I am not sure how well that would work).

Many thanks for your advice. i'll pass that on to him
atb
Dagwood :)