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A lot of our projects are CC0 licensed and I had been researching across the web to find any information regarding the legalities of this, and to retain the original author's moral rights. I have not been able to find information on the dos and don'ts of this approach though.
I think it is fine under the MIT license but I am not sure what is possible with the Apache 2.0 license.
Generally, you do not have the right to re-license the work of anyone - you have to keep at least the obligations from the license under which you receive the work.
So let's compare that:
CC0 basically is a short version of "do whatever you want, no conditions whatsoever".
MIT and Apache are both permissive licenses. Their gist basically is "do whatever you want - on the condition that you keep the copyright notice and make it accessible on all works which are a derivative of it".
So yes, you may use works under those two licenses for any purpose, and you do not need to disclose source code. But you are required to keep the original copyright under all circumstances - or you have no license to use it at all. And that's a an obligation you would remove and which you have no right to remove as you are not the copyright holder.
Conversely it's no problem if you distribute MIT-licensed content or derivatives under a GPL license (copyleft). That has the same requirement for keeping the credits intact and making them available - and imposes further restrictions on (re)distribution - something the original MIT license allows you to do. A similar argument can be made with GPLv3 and Apache2-licensed code but a bit more complicated due to some patent clauses and indemnification requirements in the Apache license - requirements which you are not allowed to remove by any relicense.
In my opinion, it is not fine to relicense content under either of those licences to CC0. CC0 includes a pretty strong waiver of moral rights, which are not mentioned in either the MIT or Apache licences. In jurisdictions which recognise moral rights and permit them to be disclaimed, you have thus exercised a right which was not given to you by the original licensor.
TLDR; You must distribute any derivative work in source code form such that the originally licensed portion of the derivative work continues to be bound by (at least) one of those two licenses.
Since you have the option of which of two licenses to use, you can choose to conform to the restrictions of the less restrictive of the two licenses. However, the two licenses say basically the same thing about relicensing. Both state that you must include that license as part of any redistribution of the work.
The Apache license says:
You must give any other recipients of the Work or Derivative Works a copy of this License
So that one's pretty clear. The way I read this, the MIT license will continue to apply to the portion of a derived source code distribution that was initially bound by the license. The MIT license takes a little more thought to come to the same conclusion...
The MIT license states that the Copyright Notice and the Permission Notice from the license must be included in any distributed derivations of the work. But if you look closely, you'll discover that the MIT license consists of exactly those two notices. By that reasoning, it also requires that you distribute any derivative works such that the MIT license continues to apply to that portion which was originally licensed by the MIT license.
So neither license allows you to remove it from derivative source code distributions, and so it must apply to the derivative work.
