Closing old lines after promo?

Last December we traded in 2 phones and added 2 lines to get one of the available trade-in promo discounts on some newer phones. I read a lot of posts here and was under the impression we’d be able to cancel the 2 old lines after 90 days. Reached out to Tforce yesterday (~100 days afterwards) and the rep said we’d lose our old phone numbers if we did so. Is there no way to change/port numbers between lines? I'd figured it would be a similar process to this: https://www.reddit.com/r/tmobile/comments/14bg22n/how_to_port_out_a_number_over_a_free_line_without/