We introduced it into our 'Maintenance' tasks (bugs, low-impact changes, etc) as a 2-week sprint. So the developers that were working on longer-term projects stayed as is, but we had a rotating Maintenance sprint. So everyone got a go at using burn-down charts and poker estimations while not disrupting the major projects.

Then as each major project ended, we started the next one using agile 2-week sprints. This whole process took a few months before everyone was on sprints, but it meant that there was less disruption & everyone could 'ease' into the process