First of all, I'm not sure if I need to replace my old Soras. I got my bike about 2 years ago from bikes direct -- it is a Motobecane Fantom CX, and I'm pretty certain that I need to replace the rear derailleur and front crank.

But anyways, when should you be sure to replace a shifter? And thoughts on micro shift?

I usually replace a Shimano shifter when it no longer works and the "WD-40" trick doesn't bring it back. Seriously, when it's completely worn out. Shifters are the most expensive part on a bike, and unless you're upgrading to 9 (if your Sora is 8) or 10-speed, it usually isn't worth it.

Sora to Microshift is only a small upgrade- I'd say my Microshift shifters are only slightly better than Sora in fit and finish and action, not as good as 105, don't know in comparison to Tiagra. They're the cheapest option for 10-speed, but they still have two levers like Sora - although the downshift lever is in a better location than Sora. They have a rather loud click when shifting, but they've been perfectly reliable for me so far (1000 or so miles). I have the 10-speed shifter shifting 9 cogs using the "alternate cable routing" method, and have found this to be just as reliable as my 9-speed Shimano drivetrains. Their ergonomics- how they feel when riding on the hoods- is about like 10-speed Shimano stuff, which is pretty good- better than the 9-speed Shimano shifters, IMO. Check back after a couple of years, and I'll be able to tell more about their longevity.

When either it's broke and can't be fixed or you're looking for a cure for upgrade-itis.

i've got micorshift on multiple bikes. absolutely love it. WAY better than sora or tiagra, probably more in line with 105 or ultegra in terms of function and it's lighter than all of them perhaps even dura ace too if your counting grams. the shifting from the drops is way better than sora but not totally ideal. The cost of microshift makes it well worth it. you can buy 3 sets of micorshift brifters for the cost of one set of ultegra. thats a no brainer to me.

What? Now they are trying to take over the shifter world!!?? Wait... you said MicroShift. I thought you said Microsoft...

Sorry, couldn't help myself.

Yep, Microshift blows away Sora/Tiagra/105 easily. They're lighter than DA7800, and they have perfectly reliable shifting. I have the 10-speed on one road bike that I use for 200k+ rides and to club race, and on my cyclocross bike. Would Buy Again.

Wear out what you have first, or just sell the bike and pick up a used one with better components.