Changing Song Words?

I was having an interesting chat yesterday with a friend about whether it's OK to change words of a worship song that's not yours. 

I remember a many years ago our church had introduced a song by a well known worship leader, but had made a slightly cheeky tweak to one of the lyrics in the version we used, just changing one word on the overhead projector. Our version had become a standard for our church, and we'd always sung it that way. I had just joined the team at HTB as a worship leader and on one of my first Sundays leading, I was using this particular song, and to my horror I spotted the author sitting at the back of the congregation, just as we got to the tweaked lyric. I quickly began to sing the right word, but the game was given away by the fact that the whole congregation sang something else! Since then I've learned my lesson! Anyway, my friend is a theologian and was raising a question he had about one lyric in a song we're currently using. His point was, could we not just change that one tiny lyric to make the song work. One example would be the famous hymn O Lord My God. I have heard that some theologians feel that the lyric 'When Christ shall come with shouts of acclamation, and take me home, what joy shall fill my heart' is weak because we really believe that Jesus is coming back here to this earth, not taking us away somewhere else. The trouble is it's actually very hard to change lyrics once a song is out there. There are a number of problems: firstly, it would be terribly confusing if every church sang slightly different versions of songs, and secondly if the song has been published, it actually becomes illegal to alter the lyrics, as they have been protected by copyright. We concluded that, tempting as it is, it is much better to (a) make sure the songs we write get proper vetting from people with a good training in theology before we launch them on the congregation, and (b) if in doubt about a song, either don't do it, or simply 'bend in love' and chose to use it, even though it's probably slightly less than perfect. We try and run all the new songs we've written past our theologian friend, but I wonder if we should be doing the same thing with new songs that we're introducing from other places too?