Can I modify the samples without the lawyers coming to get me?

Yes!

The documentation says

COPYRIGHT LICENSE:

This information contains sample application programs in source language, which illustrate programming techniques on various operating platforms. You may copy, modify, and distribute these sample programs in any form without payment to IBM, for the purposes of developing, using, marketing or distributing application programs conforming to the application programming interface for the operating platform for which the sample programs are written. These examples have not been thoroughly tested under all conditions. IBM, therefore, cannot guarantee or imply reliability, serviceability, or function of these programs. The sample programs are provided “AS IS”, without warranty of any kind. IBM shall not be liable for any damages arising out of your use of the sample programs.

Each copy or any portion of these sample programs or any derivative work, must include a copyright notice as follows:

Portions of this code are derived from IBM Corp. Sample Programs.

© Copyright IBM Corp. 1993, 2019. All rights reserved.

Note – that this does not mean you can copy the code and put it up on GitHub – as GitHub is for open source, and the MQ samples are not open source.   Thanks to Matt Leming for this advice.