How to convert specific text from a list into uppercase?

You are overdoing it, take advantage of your file structure to keep it simple!

If we find the string [United in a line, uppercasing everything from the closing brace to the end of the line gives the result you are after. Translating this into Sed language,

sed '/\[United/s/].*/\U&/' file

Note that the above is specific to GNU Sed. If not available but in a POSIX system, you can use Ex (or see αғsнιη's Awk version) with a similar syntax:

printf '%s\n' 'g/\[United/s/].*/\U&/' '%p' | ex file

To save the changes to the file instead of printing the results, change %p to x.

Using awk:

awk -F"[][]" '$2 ~/^United/ { $2="["$2"]"; $3=toupper($3); }1' OFS='' infile

within [...] we defined both ] and [ as field separator then checking against the second field if that start with United text, if yes then add back [] around second filed (I assume there is no other column inclining these characters, else those fields will miss this); then we convert third column to its uppercase value; the idiom 1 is trigger awk default print (a kind of always true condition).