Just use the following changeset:
<changeSet id="1" author="klauspfeiffer">
<sql>
insert into SEC_ROLE_ASSIGNMENT (ID, VERSION, USERNAME, ROLE_CODE, ROLE_TYPE)
select uuid(), 1, (select usr.username from SEC_USER usr where usr.ID=role.USER_ID), role.ROLE_NAME, 'resource'
from SEC_USER_ROLE role
where role.ROLE_NAME is not null;
</sql>
<sql>
UPDATE SEC_ROLE_ASSIGNMENT SET ID=REPLACE(ID, '-', '');
</sql>
</changeSet>
<changeSet id="2" author="klauspfeiffer">
<sql>
update SEC_ROLE_ASSIGNMENT set ROLE_CODE='ui-minimal' where ROLE_CODE='system-minimal';
</sql>
</changeSet>
Hopefully this will save someone some time.