It requires those contributors to license their contributions under the same license, which as a valid open source license will allow distributing all the contr

It requires those contributors to license their contributions under the same license, which as a valid open source license will allow distributing all the contributions under a larger work under that license (or compatible licenses). You're right this makes _changing_ license a lot harder, but to an extent, that's the point, as if you could easily jump from LGPL to Apache or something, there's very few ways you could make that allowance without making it easy to go to full proprietary. The DCO is basically the legal CYA your lawyers might require from contributors saying "I wrote this code any any future contributions I will submit and am happy to license it under X". Notably unlike CLA, it doesn't assign ownership to the recipient or give them permission to arbitrarily relicense.