Several states have adopted the ICC requirements for UST and AST. Not knowing where you will be doing the work, check the ICC bulletin;
http://www.iccsafe.org/Certification/Bulletins/UST-AST_EIB.pdfThey may require all technicians doing retrofitting to possess the ICC certifications before they are allowed to do maintenance at the facilities. Depending on how much work you can or want to do at these places it may be worth the hassle. It tends to be large facility maintenance companies who are part of PET who pay for their technicians to get the certification.
If you could generate a lot of work I would pursue it but not for one or two jobs unless they were paying stupid money to do it.