Bicycle Mechanics - FD adjustment going from 52/42/30 to 48/39/26

I am going to modify one of my road bikes (mostly Tiagra components) by putting smaller chainrings. Currently I have road 52/42/30 (130/74bcds) and I've already sourced appropriate 48/39/26 chainrings.

Now, the question is how exactly do I need to adjust FD? Do I really need to move it lower on the seat tube? Do I need to cut the chain? Anything else to pay attention to?

yes lower the FD to where it would be 2 mm above the big chain rings 48 and you might to shorten the chain too . you might need to readjust the FD because your new crank set may change your chain line .

Most likely, the FD will work better if you move it down.
Your current steps between rings (B>S) is- 10-12
The replacement will be 9-13, or less "linear".
This will probably result in shifting being less "precise".
Also, the "radius" of the FDER will mismatch the radius of the rings a bit more.
It should work, but be less smooth.
Don't be afraid to experiment with other than the so called "ideal" clearance above the big ring. I've had one FDER actually work noticeably better when the height was higher than "ideal".

Once you decide that you are going to keep that set up, I'd trim the chain to match.

If the front derailleur is a "braze-on" type you may not be able to lower it far enough without some creative grinding and filing. The replacement rings will work better if they are designed to work in a triple, the middle ring needs ramps and pins to assist with the shifting. I replaced the 52-42-30 rings on my wife's Ultegra triple with TA Alize 49-39-28 rings (notice the nearly even tooth count jumps) and these required lots of tweaking and adjusting to make them shift right. It's likely that you will lose some shifting smoothness when changing chainrings, especially on a triple.
After lowering the front derailleur you will definitely need to re-adjust it. Lowering it puts slack in the cable. Follow the adjustment proceedures found at http://www.parktool.com.
You may need to shorten the chain to keep it from going slack if you accidentally shift into the small to small combination.

Al

The replacement rings will work better if they are designed to work in a triple, the middle ring needs ramps and pins to assist with the shifting.

Yes, they are designed to work in the triple. 48 and 39 are from FSA, and while majority of FSA's 39 are for doubles (no pins and ramps) some FSA's 39 are for triples. TA manufacture nice rings, but they are a bit pricey... FSA is more than twice cheaper.