Migrating a subdomain from HubSpot to Squarespace (when the main domain is already on sqsp)

Hi everyone,

I need help with content migration and I will try to explain it as simply as possible:

I have two websites:

A. the main site runs on sqsps (example: waka.com)

B. the subdomain blog with plenty of content currently on Hubspot (example: blog.waka.com)

I want to migrate the content from B to A - meaning: moving all the content to A and keeping everything on the main domain +

without losing SEO rankings.

I'm not sure 301 redirects alone can even solve this, or if 301 is even an option here. 

Does anyone here have an idea how I should go around this with minimum damage?