-what akk9999 said, and:
I'd put a very visible message on all pages of old site (eg in the header) explaining that a shift will come, with a link to the root of the new site. I'd leave it there for a year, while I gradually worked on making 1-to-1 replacements for the 150 old pages.
As the closing deadline approaches, I would implement the redirects. Perhaps a week in advance, not sooner. Why? Well because you have one well ranking site, and this is the one. Don't risk that traffic before you have to.
Place a prominent link on the pages to inform your visitors about the shift. The loyal visitors must learn to use the new site first. Search Engine considerations can follow, when it is due time - no need to hurry on their behalf.
Redirects get picked up almost instantly, so no worries - Google will know. What doesn't transfer easily are rankings. So even if Google will know that this page has moved, it may not allow the ranking to move along with it, at least for some time.
best of luck :)