“Is it a difficult task to change from Win 7 to Linux ?”
It depends. One strategy is to use a minitower desktop computer for your efforts, especially if you add on a trayless hard disk drive dock in one of its exposed 3.5 inch drive bays (Bay ODD2). This approach allows you to shutdown the computer and swap the boot drive, operating system, and applications software without using dual boot or multiboot partitions on one hard drive. It also makes it easier to swap out a boot drive that has been compromised by malware and get back to work with an immediate swap in of the already prepared replacement boot drive and operating system. What this arrangement depends upon, however, is whether or not Microsoft has done any meddling with your system board or motherboard UEFI or BIOS, temporary or permanent, such as Secure Boot that is going to disable your Linux and/or do other destructive things to your computer. There are reports indicating some Samsung computers were bricked by these changes due to the interplay between Microsoft and the Samsung UEFI/BIOS on the system board. In other words, users who don’t know any better can end up having their computers temporarily disabled or permanently bricked when the Microsoft Windows 10 or recent Windows 7 and Windows 8.x updates interacts with vulnerabilities in the UEFI and/or BIOS of the computer.
How about from Windows 10 to Linux? Can I buy a Linux disk and just put it in the CD drive and have it deport Windows back where it came from?
I wish I spoke computerese. Whatever you just said sounded very smart, and was probably the holy grail of good advice.