This is a really important step and I just want to make sure I get everything right.
Basically, our current live website is Magento 1.7 and we are looking to upgrade it to 1.9. I have been working on a local version of the website (which I have upgraded to 1.9 locally) and I've just been fixing the theme due to some custom changes that were made for the current live site; and if we just went to upgrade the live site straight to 1.9, these custom changes wouldn't work and it would completely mess the site up.
What I would like to know, is once I've finished reworking the website theme on my local version, how would I go about transferring the theme files over to the live website via FTP? Like, which files should I upload? Just all the files that I've been working on? And then once the files are transferred, would I just go ahead and then update the live site to 1.9 and hope for the best?
I'd like to do this as safe as possible. I will also be making a full backup of the current live website just to be 100% safe.
Thanks for any help you can provide.
I strongly recommend that you make yourself familiar with software versioning, especially tool called git. There're several git services (such as github and bitbucket) that provide infrastructure. Use that for deployment and your benefits are twofold -- a) you save yourself from world of hurt and b) you doesn't sound like somebody from stone age.