385651 wordpress theme change

I want to change the wordpress theme/template of a site (built on wordpress) I have just acquired. Now I know that changing a theme isn't that difficult normally, just upload the new theme and you're done. However the current site has a lot of content and so I need to ensure any theme change will work properly and there will be no difficulties. You will be required to advise me and do the theme change and most importantly, troubleshoot all and any problems which may arise during this. Thanks