"Can anyone shed light on the challenges of moving a journal from restricted-access to open-access?"
Transitioning to open access has a substantial literature. An excellent analysis of the process and results is given in "What happens when a journal converts to open access? A bibliometric analysis" (https://link.springer.com/article/10.1007/s11192-021-03972-5).
"I suspect the publisher...is most interested in profitability" Isn't every publisher? Open access journals can only exist if they are profitable and many publishers - Wiley, Springer, etc - appear to be very happy with the model of charging the author to publish rather than charging the reader to read. These are highly respected high-quality publishers, but open access has actually contributed to the rise of "predatory" publishers, who will publish anything if you pay them.
And consider what happens in weak economies: their authors are intimidated by the high article processing charges that Open Access publishers demand, while their librarians are happy to soak up cost-free literature, albeit with declining input from local authors. This contributes to what has been called the "epistemic colonization" of scholarly communications.
HIFA profile: Chris Zielinski: As a Visiting Fellow and Lecturer at the Centre for Global Health, University of Winchester, Chris leads the Partnerships in Health Information (Phi) programme, which supports knowledge development and brokers healthcare information exchanges of all kinds. He is the elected Vice President (and President-in-Waiting) of the World Association of Medical Editors. Chris has held senior positions in publishing and knowledge management with WHO in Brazzaville, Geneva, Cairo and New Delhi, with FAO in Rome, ILO in Geneva, and UNIDO in Vienna. He served on WHO's Ethical Review Committee, and was an originator of the African Health Observatory. He also spent three years in London as Chief Executive of the Authors Licensing and Collecting Society. Chris has been a director of the UK Copyright Licensing Agency, Educational Recording Agency, and International Association of Audiovisual Writers and Directors. He has served on the boards of several NGOs and ethics groupings (information and computer ethics and bioethics). chris AT chriszielinski.com. His publications are at https://www.researchgate.net/profile/Chris-Zielinski and https://winchester.academia.edu/ChrisZielinski/ and his blogs are http://ziggytheblue.wordrpress.com and https://www.tumblr.com/blog/ziggytheblue