Hello and welcome to the forum.
Not sure why anyone would switch from Office Outlook to Windows 10 Mail.
Even Microsoft Office 2007 is still good through October of this year.
There is not a great deal of interoperability between the contents of a .pst file and the Windows 10 "Mail" program, which is minimal at best and which a user of Office Outlook will likely find less than adequate.
Since the Windows 10 Mail programs it normally tied to a Microsoft email account or at least an email account you have associated with your login, you can likely......
From Office Outlook, export things like Contacts and Calendar, even the current contents of a folder (like your In Box) to a file, usually a .csv (Comma Separated Value).
Sometimes it works great, other times I've found the results unsatisfactory.
In any case the ultimate outcome would depend on being able to then "import" that "exported" file into the web-based version of your email account.
outlook.com works "OK" .... I guess.
Others gmail.com, yahoo.com, etc., maybe not so much. I'm sure experiences vary.
Personally, I'll stay with Office Outlook and just keep renewing my Office 365 subscription.
Other folks seem to like other options like Thunderbird, but I have very little experience with those options.