我从2003年到O365从未做过我使用从SP 2013到O365的SMAT就像一个魅力，并考虑到你使用SMAT的数据它甚至不需要超过3到6小时取决于元数据。但是，如果您有任何自定义页面，
将会中断，即使对于付费工具也会发生这种情况。 

I never did from from 2003 to O365 but i used SMAT from SP 2013 to O365 worked like a charm and considering the data you have using SMAT it shouldn't even take more than 3 to 6 hours depends on the metadata. but yes if you have any customization pages that will break for sure this happens even for paid tool as well.