Looks like you want to change the mysite Host location to new web application(portal.company.com) and in this process you have to perform two things.
- update your user profile services for mysite host location, Create Managed Path, enable self services site creation,
- Now move all existing MySite collection from existing web app to new webApp.
To move Site collections
You can detach the database from existing(my.company.com) web application and reattach to portal.company.com web app.
Or u can take the backup of Site collections(My Site) one by one and restore it to portal web app.
helpful http://amjadk.wordpress.com/2012/12/22/mysites-and-the-mysite-host-in-sharepoint-2013/
But on the side note, it is highly recommended keep the mysite host web application separate.
Although you can use an existing web application to host these site collections, we recommend that you use a dedicated web application to improve performance and manageability.My Sites require that a site collection exist at the web application root (which is displayed as / in the user interface). Without this, you will receive a message that states that there is no site collection at the root when you try to enable self-service site creation for the web application. Because we recommend that you use a dedicated web application to host My Sites, you should use the root path for the My Site host collection unless you have a specific requirement to create the site collection deeper in the uniform resource locator (URL) path.
Read this technet for more understanding. http://technet.microsoft.com/en-us/library/cc262500(v=office.15).aspx