Read on to find out how easy it is.
In fact, this only needed a couple of minor modifications. In the non-maternity version, (i.e. as per the pattern) the front is gathered into the yoke, the seam line being just above the bust. The back is also slightly gathered, so the top is already loose fitting. I made it up in a size which is a size or two larger than my daughter would normally wear, as a pregnant bust can be a size or two larger!
Oh, and I also added 4 1/2" to the length. As you can see, the pattern has an optional extra contrasting band at the bottom of about 4 1/2". Effectively, I just added this to the pattern so I didn't have a joined on band, but just a longer top. I didn't change the back.
The other small change I made (which is not really to do with making it into a maternity top) was that I shortened the straps front and back by half an inch, and cut the neckline a bit higher at the front. I actually used the size smaller cutting lines to achieve this. This is because my daughter is not keen on low cut tops, especially while pregnant, and especially for work.
This top has optional sleeves, but they are fussy frilly sleeves, which I knew she wouldn't like.
It works fine as a sleeveless top. In the winter (i.e. up till now) she can wear it with a long-sleeved T shirt underneath, which still fits round the top but which, on its own, would increasingly reveal her expanding girth. It will also work underneath the waterfall cardigan or wrap that I made her. Then in summer - if she makes it to the summer - it can just be a nice lightweight sleeveless top. I'm not sure if it will take her all the way through, but it will do for a few months yet.
It looks better on than off - need to get an 'on' picture! The pattern is so strong that you can't really see the yoke seam.