I knit this sweater t-shirt 3 years ago, it's called Miley tee. I think I've worn it all of two times. The reason being it sat shorter than I liked and the neckline kept slipping backwards down my back. For years I've thought about fixing it. Since it was knit from the top down, this is not a problem, at least for the length that is.
To remedy the length, it was a simple matter of undoing the bind off. Thankfully, it was easy to find the tail and pick it out. Oh, did I mention that I picked up an extra skein of the yarn? Silky Wool (DK: 45% Wool, 35% Silk, 20% Nylon, 192 yards). The dyelot was ever so slightly different so I alternated rounds between the working yarn of the sweater and the new skein. Given that it is top down, I was able to try it on to determine the desired length. Yes, in answer to your question, I did this when I knit it and thought it was fine. Bygones. I added about 3" in length.
Since I had leftover yarn, I decided to lengthen the sleeves; from the school of "If you give a mouse a cookie" otherwise known as the "while we're at it" factor. I weighed the remaining yarn on a food scale and split the yarn evenly into two balls.
The sleeves were less forgiving in finding the tail of the bind off. I wove it in to damn well; I had to snip the bind off. I hated to do it, but I figured I could use the little bits for fixing the neckline.
The sleeves needed a good 4" more to bring it past the elbow. A far more attractive place on my arm, I can tell you that much! I also decided to decrease the sleeve width by an inch's worth of stitches. In order to do the math, I had to follow the following steps (I actually wrote it down to share):
1. measure stitch gauge. 5 sts/1" round up to 6 sts.
2. measure row gauge. 10 rnds/1"
3. 6 sts/2 = 3 decrease rnds (keep it even).
4. 10 rnds x 4" = 40 rnds.
5. 40 rnds/3 = decrease every 13 rnds 3x.
When I finished the first sleeve and bound off, I didn't fasten off in case I didn't have enough yarn for the second sleeve. Allowing for Murphy's Law and not trusting the scale (must be a female thing). Thankfully I did have enough, with some to spare. Somewhere a long the line a random little ball of yarn appeared. Must've been yarn elves.
Now, the neckline. The main impetus behind finishing the sweater now versus anytime within the past 3 years was a little helpful hint I learned from Chris Bylsma in a class I took at TNNA (The National Needlework Association). She explained that when you have an especially wide v-neck, the back of the neck will slip backwards. To fix that, she suggest working a slip stitch chain along the base of the neck. Brilliant.
That's exactly what I did.
Post Script: The sleeves were a little shy of where I wanted them to fall and I did have that extra bit of yarn. I lengthened the ribbing with what I had left using up practically every last bit of the yarn.
Meanwhile, the beasty boys basked in the sun.