gistfile1.txt

# start clean
rm(list=ls())

# load libraries
library(data.table)

DT <- data.table(A = rnorm(10), B = rnorm(10))

# this gives lots of warnings, there seems to be a better way of doing this
names(DT)      <- tolower(names(DT))

# same with this line
names(DT) <- gsub(" ", ".", names(DT))

The error message says that it's better to use setnames() instead of names<-. This was meant for older versions of R which copied the entire data.frame/data.table to just replace names. Improvements were made in Rv3.1+. But still the idiomatic way would be to use setnames().

HTH