The banners and shields will display the emblem of the guild you are in when placed, and will not change automatically when you change guilds. In order to change the emblem to that of your current guild, you can use the command /house fixemblems.
In case that the emblems do not update after using the command, you may have to manually exchange them. You can select 'interior shield removal' and 'interior banner removal' from your House Interior Options panel and buy. Then, purchase your new shields and banners from the same panel.