I think you'll need to replace all of the elbows, tees, etc that you're showing in the picture, with the possible exception of the elbow connecting the funny pipe (that's what they call it here; I don't know what the real name for it is) with the PVC. I have no idea how much it will cost for a pro to do the repair. The biggest part of the cost would be the visit and basic charges. For a pro, there's not that much difference between replacing a sprinkler head and replacing all those pieces. Of course, you have to pay for all the parts he'd supply, but I still think the basic visit/work order part will be the biggest piece of the bill.

When I purchased my home, (built in 1991), I quickly discovered that when the sprinklers were used they would suck all the water out of my pipes and I would get air through every faucet. I installed a brass check valve in the main line near the sprinkler pump. Now I never lose house water, but I also have NO pressure when they are on, so I must water at 4AM. I will not go the expense of a tank to maintain pressure.

I would figure out a way to get rid of the galvanized pipe. If it is very old, it will probably be corroded inside and out and may have restricted water flow. Consider using pipe larger than 3/4" because it should improve the water volume. My pipe is 3/4" copper at the meter and I used 1" PVC for all my lines right up to the heads. First though, I set up a 3/4" section and then a 1" section to see if it would really help. The 3/4" pipe had a volume of 8 gpm. The 1" pipe has a volume of 13 gpm. I measured this with the Toro pressure and volume gauges. Needless to say, I went with the 1" pipe all the way and only reduced it right at the heads. Used 1" brass valves too. This system really puts out the water. Yard looks like it is flooded in some areas and I am using 7-8 heads per valve. As far as removing the galvanized pipes, it would probably be worth seeing if a concrete cutting company could cut around them to get them out. The resulting holes should allow enough room to connnect PVC under the walk. If it is just one section of the walk, the alternative might be to remove that section of the walk, install the PVC and repour the walk. Might be cheaper than a concrete cutter. If you don't remove them, a few years (or less) you will probably have reduced flow or leaks or ?? I would rather deal with it now.

I have an existing sprinkler system on the raised vegetable bed that is not efficient. I want to convert it into a pvc system using risers to replace the sprinkler heads. I tried once to replace the sprinkler heads with the adjustable manifold, but the connectors to the pipes kept breaking off. Anyone tried this?