Migrating from SP2016 to cloud: Modern vs. Classic

Question

My client (a U.S. Navy command) is currently using SP2016 On-Premises. They have been told that SharePoint Online will soon be available for them to migrate to. I have read about Modern vs. Classic, and they would prefer to remain on Classic. My question is, does Microsoft impose the Modern view in any way when a SharePoint Online site collection is created? Can the decision to be Modern vs. Classic be made at the site collection level? At the site level? At the page level?

Let's say the Navy tells Microsoft, give us 100 new site collections. If the majority of them are Modern, is there any pressure for ALL of the new site collections to be Modern, or does anyone really care?

Is there an article that addresses this? And please, don't reply with a "compare Modern to Classic" article, I have seen them, and know the differences, thank you. This question is specifically about remaining Classic when other site collections are going to be Modern.

Answer 1

Yes, it's possible to define Classic view in O365, if this is your requirement; recently (last month), I have some demand with similar requirement, moving one Site Collection from SharePoint 2013 OnPremises to Online, and I get these conclusions:

1. In branding: master pages, style library, imagens, needs attention for relative URL references. The styles with some updates in core.css, it was possible to moving without effort, but some others don't (this applies for javascript too);

2. For Lists: you can move to classical view at Site Content page (for Site Content pageview) and when need to edit NewForm.aspx adding some JavaScript to apply some UI rule;

3. I've tried several resources to move Infopath forms, include non supportable alternatives and I cannot do it;

I think that's all.