With UNRELATED employers, I think your answer is yes, that Plan A can reclassify deferrals as catch-up, as long as his catch-up limit under Plan A is not already used up.
First, the individual deferring $20,500 between the two unrelated employers' plans doesn't cause catch-ups in either plan. From the preamble to the 414(v) regs:
QUOTE
F. Excludability of Catch-up Contributions
JCWAA amended section 402(g) to increase the elective deferral limit for a catch-up eligible participant by the amount of the allowable catch-up contributions for the taxable year. The provisions of these final regulations related to these provisions are under new §1.402(g)-2, rather than under §1.414(v)-1, as in the proposed regulations. Under §1.402(g)-2, the amount of elective deferrals that a catch-up eligible participant is permitted to exclude from income under section 402(g) for the taxable year is increased by the maximum amount of catch-up contributions permitted for the taxable year under section 414(v). This treatment by the catch-up eligible participant is not affected by whether the applicable employer plans treat the elective deferrals as catch-up contributions. Thus, a catch-up eligible participant who participates in plans of two or more employers is permitted to exclude from gross income elective deferrals that exceed the section 402(g) limit, even though neither plan treats those elective deferrals as catch-up contributions. In addition, the treatment by an individual of such elective deferrals as catch-up contributions will not have any effect on either employer's plan.
Then, look at
QUOTE
1.414(v)-1(a)(1)General rule. --An applicable employer plan shall not be treated as failing to meet any requirement of the Internal Revenue Code solely because the plan permits a catch-up eligible participant to make catch-up contributions in accordance with section 414(v) and this section. With respect to an applicable employer plan, catch-up contributions are elective deferrals made by a catch-up eligible participant that exceed any of the applicable limits set forth in paragraph (b) of this section and that are treated under the applicable employer plan as catch-up contributions, but only to the extent they do not exceed the catch-up contribution limit described in paragraph (c) of this section (determined in accordance with the special rules for employers that maintain multiple applicable employer plans in paragraph (f) of this section, if applicable). To the extent provided under paragraph (d) of this section, catch-up contributions are disregarded for purposes of various statutory limits. In addition, unless otherwise provided in paragraph (e) of this section, all catch-up eligible participants of the employer must be provided the opportunity to make catch-up contributions in order for an applicable employer plan to comply with the universal availability requirement of section 414(v)(4). The definitions in paragraph (g) of this section apply for purposes of this section and §1.402(g)-2.
Since you have unrelated employers, the catch-ups under Plan B would not be catch-ups with respect to Plan A. The Participant would have to hit some limit under Plan A before he has catch-ups under Plan A.
Note, if A & B are RELATED employers, catch-ups would be determined by treating all of their plans as a single plan. In that case, your answer would be no.