coverting cell to number without it deleting zero's

Hi Guys,

I have column of numbers used as item codes, they appear with a small green triangle in each cell.

When I select all cells a little blue square pops up with drop down options to convert to number- after doing this the green triangles disappear but then numbers I had listed as eg '043340' appear as 4334.

Is there way to make sure zero's are not deleted as some of my results from formulas end up being incorrect.

Thanks in advance for your help

Cheers,
Natalie