In the consumer space, a repair where PbFree is removed and 60/40 is used is generally not an issue. The product was dead/faulty anyway necessitating a repair, so any and all the extra life provided by a traditional repair is good for everyone (and the environment).
Some info on that from a few "industry experts"
http://www.circuitnet.com/experts/39990.html
It comes down to making sure that there's an even distribution of both the lead-free and the SnPb in the repaired joint.
For us in the mil world it gets even stranger, since as the first respondent mentions we are soldering BGA's with SAC balls (ok I know..) to PCB's that are SnPb SMOBC/HASL and using SnPb pastes. The CM's I've talked to mention the same thing - to make sure that the reflow profile is the adequate for both materials.
As to rework - make sure you're using a proper soldering iron temperature. In the hundreds of reworks I've had to do for clients the biggest issue is the No Clean flux mixed with something like a water-soluble. Gets nasty...
On the topic of environmntal - it might be that Lead Free is just as bad for the environment: this is and older post from Dr Howard Johnson:
http://www.sigcon.com/Pubs/news/10_01.htm
You'll see a section :
Myth #8. Eliminating lead from solder saves the environment.
"According to John Burke at the UCSC Extension in Silicon Valley (see Managing Environmental Change in Electronics) the US Environmental protection agency "Report on Solders in Electronics: A Life-Cycle Assessment" (472 pages) published August 2005 shows that the replacement for “leaded” solder generally referred to as “SAC alloy” has a greater impact on the environment than tin lead solder in a number of areas such as non-renewable resource use, energy use, global warming, ozone depletion, and water quality. Admittedly, leaded solder performs worse on issues of toxicity, mainly to workers who handle the material. Manufacturers can (and have already for the past 60 years) addressed the toxicity issue for workers by using exhaust hoods, wearing gloves, and washing hands. My conclusion? SAC solder is markedly worse for the environment than lead-based solder. We are not saving the environment by moving to SAC solder, we are damaging it."
Another article from 2011 states as similar thing
https://www.ecnmag.com/article/2011/12/was-lead-free-solder-worth-effort
From that article:
"To delve deeper into the need for lead-free solder I talked with a knowledgable person at the IPC, formerly know as the Institute for Interconnecting and Packaging Electronic Circuits. When I asked about any follow-up studies that could show the benefits of lead-free solder, the spokesperson explained that such studies would simply waste money because they would show no changes in lead exposure in landfills and no changes in occupational exposure, either. Years ago companies recognized the dangers of lead exposure and took measures to protect employees. The problems in third-world salvage yards stem from improper waste management.
The IPC person also explained the EPA produced an overall hazard assessment that included aspects such as ozone depletion and global warming and did not provide an exposure assessment. So far, the EPA has done no additional research to determine the effects of eliminating lead from solder.
In the end, though, lead-free solder proved beneficial for solder and soldering-equipment vendors, but overall, the entire effort seems like a waste of time, talent, and money. "
And search for the word electronics in this article:
https://www.epa.gov/lead/learn-about-lead
You won't find it.
Not sure about this but I recall reading in one of the scientific journals that the study done that supposedly shows lead leaching from landfills from discarded electronics used a a bit of a faulty method of pulverizing the electronics - which would not naturally occur; recall plastics and other materials that are used to contain most electronics are long-lived.
