Hi, Spidergirl,
I just asked my regulator (FDIC) this same question as it pertains to loans (not Lines of Credit). Their response is that you would NOT report the March increase because the original loan is over $1MM. The same would appear to go for the $250,000 then as well.
If, however, you have a loan of $600K in Jan of 2018 and an increase of $500,000 in May of 2018, you would report both loans on the CRA LAR because you can only report an increase if the original amount has already been reported one time that year. (If the original loan is over $1MM, it would not have been reported, therefore, the increase cannot be reported.) I would not report the $500K as "other loan data" as mrogersfib suggested as I believe they are referring to situations where increases to Lines of Credit put the total loan over $1MM.
I hope this helps!