As I thought it may be useful to some one in my shoes, I am posting below what I did.
To change to SSD before switching over to Windows 10.
1)Selection of SSD
Search was made for brand and size. After lot of search in may sites regarding performance and comments by users, I decided to go for Samsung. Then I have decided to go for Evo than Pro as Evo has performance nearer to Pro and good value. Then size is decided as 250GB as SSD at US$100 and external WD passport ultra 1TB at US$60 had more working and backup space than Evo 500GB at US$165.
2) Method of change to SSD
Whether first migrate data to SSD or Change over to SSD and then migrate data or change over to SSD and install new windows 8.1.
As original installation was windows 8 and then it was upgraded to windows 8.1, factory reset leads to windows 8 only. Further 30 months use laptop with various software install and uninstall, I thought it is better to go for new installation of windows after putting SSD. After browsing, I found that this is possible as key for windows 8 is stored in UEFI BIOS. However in case any problem crops up, I wanted to go for old installation. Hence after lot of research, decided to for free Macrium reflect to do backup image and rescue USB Stick. Then made image and rescue disk. Then I downloaded windows 8.1 ISO using Create installation media for Windows 8.1. I mounted ISO and copied files to fat32 formatted USB stick of 4GB.
Process of installing SSD and new windows.
Here I followed You tube video given below; After installation of SSD, I booted from USB and installed windows 8.1 without any problem with my Microsoft account.
Use of image.
After installation, I found that I should have made back up personal.xlsb file and also windows wallpaper background folder. Here Macrium image came useful. Once Macrium reflect is installed, image can be browsed and file or folder can be copied. With this facility, I have recovered my Personal.xlsb and background folder.