Differences are many. Generally, the only code that currently exists/shared between the two is the code that scans the post/page content to generate the TOC. Even in that code there have been changes, bug fixes and filters added for other devs.
ezTOC does not support TOC+ shortcodes such as [no_toc], [sitemap], [sitemap_pages], [sitemap_categories] or [sitemap_posts]. These were removed to focus on TOC only features.
The most recent version of ezTOC did add the support for the [toc] shortcode to help those who wish to switch plugins but none of the TOC+ [toc] shortcode options are supported. It is only used for placement of the TOC on the page. That said the same options, and more, are supported as settings found on the post/page when editing the page/post in the admin. Generally speaking, I am not going to invest much time in supporting the shortcode as I wish users to use the Global settings found under the WP Settings menu tree and the settings found on the page/post when editing a page.
ezTOC does have include all the features that existed in TOC+ in regards to the actual TOC plus a few minor additions such as replace a header with an alternate header TOC list item. Meaning a header in the post body can have alternate text displayed in the TOC list. Useful for when you have a long header and wish to have a more concise link in the TOC.
Can you simply switch from TOC+ to exTOC? Really, that depends. If you only used the [toc] shortcode with no options and do not need the or have not used the other TOC+ shortcodes, then yes I would say you can easily switch.
Hope I have answered your question thoroughly!