Hello. Changing the keywords if you do not change the content of the page and title will make no diffrence since Google do not take keywords into consideration. But if you change keywords and content, you will lose all the rankings you had previously. I advise to not make changes to keywords very often since it will take forever to reach and rankings.
Better to just add more content with the new keywords you want to target. Focus your SEO structure around those if you are attempting to rank for the new keywords.
I really agree with you and i think you given best answer of this question. Yes, if you add keyword on Title Tag than you have to add also in content.
I agree with above responses. Adjust with more content and keyword-backlinks. If they are just additions or slight changes to existing keywords then you'll be fine.
you'll lose all the earlier rankings..so its better to add more stuff with new keywords which you want to use..
By the way i'd like to recommend a good keyword research tool - semrush, it will help you to find necessary keywords..