I am looking forward to building my own website but the name is not final as yet and might change based on the name of my small buisness. Can I change the name of website at a later stage like I buy http://www.abc.com and change it to http://www.pqr.com later with the WordPress premium plan?
You can change it later, yes, but the way you change it would be to buy the new domain separately. Custom domain names are registered for a year at the moment you buy it, and it cannot be edited, only replaced. As your plan only includes one domain for free, any further domains you add on top of it will cost extra.