Using 變音 when reading Classical Chinese poetry in Cantonese
It should be read in the original lau4 (陽平) tone, but not only because it's "classical", but primarily because the poem follows the tone pattern of a five-syllable regulated verse (五絕).

Tones in Middle Chinese were classified into 平 (level), 上 (rising), 去 (departing), and 入 (entering), the latter three of which were classified as 仄 (oblique) for the purposes of tone classification in a regulated verse.  平 tones correspond to modern Cantonese tones 1 and 4 in the 9-tone system where entering tones are numbered separately, and 仄 correspond to the rest.

A typical four-line five-syllable regulated verse that starts with a 仄 tone syllable (of which 《登鸛雀樓》 is one) has the following tone pattern:


  仄 仄 平 平 仄
  
  平 平 仄 仄 平
  
  平 平 平 仄 仄
  
  仄 仄 仄 平 平
  
  Notes:
  
  
  The syllables marked in bold are those that can be substituted with their "opposite" tone (i.e., 仄 instead of 平 or vice versa).
  The first line may also be 仄 仄 仄 平 平.
  All 平 syllables at the end of each line must rhyme.
  


The line that you cite, 更上一層樓, is the last line in the poem. The Cantonese tones of the first four characters in this line is 3-5-7-4; you can see that the tone of the last character must also be 4 in order for it to match the tone pattern 仄 仄 仄 平 平.

EDIT: I found a YouTube video as well where you can clearly hear it pronounced as lau4: http://www.youtube.com/watch?v=6YakQ1ACaf0