Why promote language standard from -std=gnu++98 to -std=gnu++11

I have a relatively well-maintained, rather large code base using -std=gnu++98. What are the advantages of upgrading the language standard from gnu++98 to gnu++11 ? What are the risks ?

If there are advantages, is it enough to just change from -std=gnu++98 to -std=gnu++11 ? Are there other options that should go along with this change (thinking about how -pendantic -pedantic-errors accompanies -std=c89 or -ansi for C)