I looked through the scientific literature for calcium aluminates and I did not find any evidence that monocalcium aluminate decahydrate(CaO.Al₂O₃.10H₂O) will add water to become monocalcium aluminateundecahydrate (CaO.Al₂O₃.11H₂O), so I'm not sure there is an answer to your question. The only undecahydrate I found in this chemical family was Ca₄Al₂O₇.11H₂O which is not the same compound you asked about.

Most calcium aluminate hydrates have a hexagonal structure. The powder diffraction patterns for any hexagonal structure will be very similar with only, as you say, a slight difference corresponding to differences in d spacings. If changing the hydration of the compoundchanges the overall crystal structure, then you will get an entirelydifferent pattern, as in the case of Ca₃Al₂(OH)₁₂ which has a cubicstructure.

References:
Edmonds, R.N.; Majumdar, A.J. "The Hydration of Monocalcium Aluminateat Different Temperatures." Cement and Concrete Research. 1988, 18,311-329.
Jensen, T.R.; Christensen, A.N.; Hanson, J.C. "Hydrothermal transformation of the calcium aluminum oxide hydrates CaAlsO₄.10H₂O and Ca₂Al₂O₅.8H₂O to Ca₃Al(OH)₁₂ investigated by in situ synchrotronX-ray powder diffraction." Cement and Concrete Research, 2005, 35,2300-9.