So, this happened to me again, thankfully this time on personal art wheeeh! I was drawing a piece for huge printing but it was only 2000x2000 but needed to be 4500x4500 instead! Uhhh!
This will only work if you're looking to have sharp lineart, it wont work on textured lines or stylishly blurry / partly transparent things. I use Krita 2.9, similar tools are available on other programs but they may function slightly differently!

1. Upscale piece, better results potentially come if you upscale into something very huge (then you can always downscale after).
2. Merge black lines with a white background layer
3. Take bucket fill and set it on a high threshold (I used 70) as white colour, but no grow or feathering!
4. Apply on white space, it'll eat up some of the gray blur around your lines! I applied the effect twice on all spots!
5. Filter > Colours > Colour to alpha on white, so lines are on transparent again. And there's that! :D

Before and after comparison on 200% zoom, up side of line is sharpened and underside is original upscale:
http://neotheta.fi/s/Selection_065.png

Results viewed on 100% (real) size:
http://neotheta.fi/s/Selection_066.png

Now only smallest details need some refining, but no need to redraw yay! Hopefully you found this useful too :3
*Neotheta submits their journal on read-only mode*