Yes. When you elect to change from a Core portfolio to an ESG portfolio, Acorns must sell the ETFs underlying your Core portfolio and then purchase shares in the ESG ETFs.
The sale of securities is a potential taxable event and you may want to discuss with a tax professional whether changing portfolios is appropriate for your unique tax situation. This is not tax advice and Acorns does not provide tax advice. Please contact a tax professional if you have questions.