Will switching from Minoxidil 10% to 5% increase hair fall?

Minoxidil affects everyone differently. While minoxidil works fine for the
select few, the problem with the medicine is that it does not actually
stop hair loss. Instead, DHT will continue is binding to the hair shaft
which will eventually overtake any additional growth caused by the
Minoxidil and die, perhaps just slightly later than it would have
without the medicine.
Also, the alcohol base required to
support the active ingredients also cause irritation of the scalp and
severe reactions for many people, such as the increased shedding you are
describing. The fact is that it does not work for everyone, and even if
it does, not for long and not without side effects.
The number one first step I recommend
is a comprehensive consultation with a hair regrowth specialist who
will perform lab tests to determine the underlying cause of your
thinning. Without labs, no-one can really know what your true cause is
and any treatment will be temporary at best.
From here, you can receive the best treatment for your case.
Good luck on your search.

You shouldn’t notice much of an increase just from switching. However, long term minoxidil is not always the best defense. Using Finasteride could possibly give you better retention of native hair. Have your hairloss specialist go over all options that may benefit you long term.