I plan to change templates to certain blogspot sites? Will the change destroy my Page Rank, SERP ranking?
Yes, you can expect to see a drop in SERPs for a decent amount of time. Less if you keep the same basic content layout and just alter the design itself, in fact if you don't change the actually content displayed on each page or re-arrange it you won't see as much a drop, this is usually not the case with templates usually tho.
I understand that if you change the template (templates not from blogger itself), the widgets and sidebars will be wiped out so I have to reinstall them again. Also, I plan to change from three columns to two columns. The content is still the same but the column structure and template style would be revamped. But I just can't take it if the SERP would drop significantly.
Yes, because your changing the web page, it might be a small alteration but your still making a change.
Changing the template can have an effect on your SERPs, usually template changes will change the structure, they go hand in hand. If the template has the same structural internal linking from the front page you still might see some minor changes for a small amount of time.
but hey i want to ask that can the effects be positive too???? like you changed template and jumped up in SERP??