At RiverGlide, we’re often asked to help with “Agile transformations”.
My definition1 of agile is being flexible to adapt an existing process.
That is, if your existing process looks like waterfall, you’re still agile if you introspect, retrospect and experiment to improve your process. If at the end, your process looks like waterfall, then as long as you have introspected, retrospected and experimented and found that this approach gives the best results, then your process is agile.
 Although this might not fit with other definitions. Your mileage may vary